The obvious thing to say is that it depends on the gig. To elaborate beyond that, I'd say it depends on how you use it, and particularly the extent to which you "play the pedalboard" within any given song. Doug Wimbish will have something on the order of a pair of 4'x8' pieces of plywood covered in pedals and he's dancing all over the things, playing his pedals as much as he's playing the instrument on a strap. In a sense, his pedals ARE his instrument and the bass itself is just their trigger, so he's got the whole huge spread of them.
In Mugshot Monday, I have some songs where I play the pedalboard a little bit - there are effects that come on and off in different parts of a song. So the board needs to be big enough for those pedals, but small enough that I can easily reach them (especially since I'm often at a mic singing at the same time).
This is what I'm normally using:
View attachment 5196441
If the use of effects
within any given song is set-it-and-forget-it, though, the board can be as big as you like, because you don't need to worry about being able to reach any given item mid-performance.