Depends on what type of pickup your using, active with a pre-amp, any 25-100k will do, mag pickups like a single or hum-B 250k for singles and 500k for hum-B, keeps from losing the treble side of it. Too low a resistance with a mag and after you barely start to adjust the pot, you pretty much lose any real adjustment before reaching halfway, different pickups, different pots, many variations depending on what your trying to achieve, but what I gave you isn't a rule, but a basic guideline used for the most part.
EDIT ***sorry, basically advice I gave is for the guitar itself, for the amp...not sure, higher resistance gives a wider range of adjustment.
Others will chime in.
For the amp or for the guitar?
Most small amps use a 10k to 50k for volume. Ruby amps, Smokey amps, Little Gem and Noisy Cricket amps all use a 10k pot for volume.
Guitars will use 250k for most single coil pickups, 500k for double coil humbuckers and some large single coil pickups depending on your tastes. Piezo's can use 500k to 1meg.
I'm working on an amp