Mathematics tells us that when you downtune a whole step, you lose about 11% in total string tension. When you then downtune the bass string by yet another whole step, that string has 21% less tension than when tuned to E, which shouldn't be surprising, it's only a half step away from B in a 5-string. Ideally, then, you should be using much heavier strings. I feel like a standard set of light bass strings would be something like 45-60-80-100. If you're used to that, then for standard D, you should be using 50-70-90-110 to have about the same tension. Ideally, a drop C set would then be 50-70-90-120. However, if you're not always in drop C, you might find 120 a little too stiff when tuned to D (and especially to E). A 115 would split the difference, so you might look at 50-70-90-115, or if you like a slightly slinkier feel, 50-70-85-110 or something like that. Websites like Circle K or JustStrings will let you buy single strings to do exactly that.
Keep in mind, once you're set up with heavier strings, tuning up to Standard E will make the strings feel quite stiff, that's why 50-70-85-110 might be a good idea so that you kind of strike a balance that works well for everything.