Most player spans are irrelevant in measured size, because a measured span does not account for functionality.
A player should have a stress free two tone spread on their bass, that's a comfortable two tone spread free of tension on the back of the hand.
So even with the fore finger on the first fret 'F', there is no need for the little finger to cover the fifth fret 'A' the open 'A' will achieve a two tone coverage rather than a fifth fret 'A' if need be...but a slight pivot or slide of the hand can be used if that fifth fret 'A' is what you desire.
The same also applies 'F#' - 'A#' and 'G' -'B' and so on.
They should be attainable with a slight pivot or slide...but again it should be stress free.
That area of stress and tension is on the back of the hand not the front.
As many never really exercise the extensors fully, this musice group that pulls the fingers back and open becomes tight and restricted, but exercise them and your span will increase, remember that is both hands.
Your plucking hand extensors would benifit from exercise a well.
As for me...i have a comfortable two tone span, that is more than functional for me.
A great exercise for anyone looking to see the functionality of their fretting hand is this one.
Play four chromatic notes one finger per fret, so lets take at the 7th fret 'A' string, E F F# G.
The next chromatic note will be 'G#'.
Rather than spread the little finger up the neck to it, hold that 'G#' postion and spread the forefinger back and play 'G#' 6th fret D string.
Then repeat the action and play G# A A# B, then go back and play C 5th fret 'G' string.
You can use this frame work all over the fretboard to play chromatic scales forwards and backwards up and down the neck.
The fact you have to make spans will stretch your soft tissues and increase your span over time with practice....again all this should be stress free. Any further spans are really not needed because a standard bass is tuned in fourths, a two tone span should work great.
Those looking to play chords have a different set of criteria in their function, but that said, the playing of chords will in itself exercise the hand and increase span.
In both senarios it is important to exercise the hands in a way that fully opens and spreads them, this allows the joints to move in the correct order and in the correct way.
Full finger speads and fans are great exercises, as are finger spreads and warm ups. For details of some great basic ones check out the link.
Any questions that this throws up, post them or PM me and if i can answer them i will reply.