There are a lot of different thoughts on string length and string instruments. While we are all familiar with the violin family and the debate about whether or not the double bass is a part of it or not, there are other string instruments out there as well that explore the difference in string length and body size in relation to pitch. The viol family debatably has seven different sizes from the pardessus which is about the same size as a violin with a high g string (which would be a minor third above the open e of a violin) and the bottom string is a low G the same as the violin. Some of them are only 5 strings, starting on a C a fourth above the open G on a violin. They reach down to bass viols that are similar in size and range to a cello, and then to what are sometimes referred to as contrabass viols or violone, with the violone in D (the bottom string is a tone lower than the double bass E) ranging anywhere from slightly smaller than a 3/4 size double bass to slightly larger than a 4/4 sized orchestra cannon. The Violin Octet/Hutchins violin family expands the traditional violin family, and the contrabass violin was designed to be tuned as a bass in 5ths (the same octave as a double bass in 5ths) but is sometimes played in 4ths so bassists used to playing in 4ths can make the transition easier. It typically has a larger body size, a 43" string length, but is shaped like a violin with narrower ribs and broader shoulders.
Various studies have been done to try to determine what is the optimal string length and body size for specific tunings. It is largely agreed that the viola should be larger than it is to work properly, and a 17" or even 18" string length would be optimal. The Violin Octet Alto Violin (the same tuning as a viola) has a 2-3" larger body size and is played vertically like a cello, but often has a string length of 16 3/4" or so such that violists can transition easier. Likewise, there are people that think something around 45" would be the optimal string length with a 5/4-6/4 size body, but few bassists could play on such an instrument. The Octobass was also flirted with for a while and was seen as the answer to a more powerful low end in the string section, but that didn't really take off.
All of that context largely to point out that our standard 3/4 size 40-42" double bass is "too small" both in comparison to the size of other string instruments, and according to some acoustic/other scientific studies. The double bass is full of compromises in order to make it a manageable instrument for the player and to meet the demands of the repertoire we play on it. Many large instruments have either been reshaped, had their string length reduced, or both, and a lot of modern makers are trying to find a way to make instruments that look traditional, but are easier to get around. This often manifests as sloped shoulders, a smaller upper bout with a larger lower bout, a big body with a not so long string length, canted back, narrow ribs or ribs that taper, or a combination of these things. There are some players that just cannot play a larger instrument/string length with their current physical or technique limitations, and others who play instruments that seem too large/small for them and manage just fine. Thomas Martin for example is 5'5" and plays on huge instruments just fine. It is also worth mention that while generalizations can be made about bigger or smaller instruments and longer and shorter string lengths, there are some fantastic (and horrible) exceptions to that rule, with small instruments sounding huge and huge instruments sounding wimpy.
Without getting into the 4ths/5ths debate, there are some who play in 5ths and some who play with "4 finger technique" where each finger gets a semitone in either tuning who like a shorter string length. There are others who are happy with 41-42", a few who play on something longer, and some who believe that with pivots and all four fingers, string length becomes irrelevant and any string length is possible.
Eventually, it comes down to personal preferences with a heavy amount of influence from our current conventions. 41-42" string length on a 3/4 instrument has become our "standard". There are some players that are looking for something longer or bigger, but these tend to be the exception not the rule. Anything below about 40" is usually considered short, and all of the stereotypes about having no bottom end, not carrying to the back of the hall, being a "solo" sized instrument that sounds like a cello etc. get thrown around. Anything longer than 42" and it's massive or unplayable, but it likely sounds like a cannon that was shot off three towns over. Sometimes these things hold true, and sometimes they aren't worth the paper they're written on. Or whatever the internet equivalent of that saying is.
Various studies have been done to try to determine what is the optimal string length and body size for specific tunings. It is largely agreed that the viola should be larger than it is to work properly, and a 17" or even 18" string length would be optimal. The Violin Octet Alto Violin (the same tuning as a viola) has a 2-3" larger body size and is played vertically like a cello, but often has a string length of 16 3/4" or so such that violists can transition easier. Likewise, there are people that think something around 45" would be the optimal string length with a 5/4-6/4 size body, but few bassists could play on such an instrument. The Octobass was also flirted with for a while and was seen as the answer to a more powerful low end in the string section, but that didn't really take off.
All of that context largely to point out that our standard 3/4 size 40-42" double bass is "too small" both in comparison to the size of other string instruments, and according to some acoustic/other scientific studies. The double bass is full of compromises in order to make it a manageable instrument for the player and to meet the demands of the repertoire we play on it. Many large instruments have either been reshaped, had their string length reduced, or both, and a lot of modern makers are trying to find a way to make instruments that look traditional, but are easier to get around. This often manifests as sloped shoulders, a smaller upper bout with a larger lower bout, a big body with a not so long string length, canted back, narrow ribs or ribs that taper, or a combination of these things. There are some players that just cannot play a larger instrument/string length with their current physical or technique limitations, and others who play instruments that seem too large/small for them and manage just fine. Thomas Martin for example is 5'5" and plays on huge instruments just fine. It is also worth mention that while generalizations can be made about bigger or smaller instruments and longer and shorter string lengths, there are some fantastic (and horrible) exceptions to that rule, with small instruments sounding huge and huge instruments sounding wimpy.
Without getting into the 4ths/5ths debate, there are some who play in 5ths and some who play with "4 finger technique" where each finger gets a semitone in either tuning who like a shorter string length. There are others who are happy with 41-42", a few who play on something longer, and some who believe that with pivots and all four fingers, string length becomes irrelevant and any string length is possible.
Eventually, it comes down to personal preferences with a heavy amount of influence from our current conventions. 41-42" string length on a 3/4 instrument has become our "standard". There are some players that are looking for something longer or bigger, but these tend to be the exception not the rule. Anything below about 40" is usually considered short, and all of the stereotypes about having no bottom end, not carrying to the back of the hall, being a "solo" sized instrument that sounds like a cello etc. get thrown around. Anything longer than 42" and it's massive or unplayable, but it likely sounds like a cannon that was shot off three towns over. Sometimes these things hold true, and sometimes they aren't worth the paper they're written on. Or whatever the internet equivalent of that saying is.
