for most guitar shops around, you can only find 250k, 330k(maybe) and 500k.
For guitar purposes, you wont find much place selling 1meg pot. For general purpose pot from sim lim, then yeah, you can easily find 1meg, 2meg etc. But those are not for guitar purpose, and quality suffer, in term of value accuracy and taper.
And the bigger value pot you use, it will have more prominent effect when use as volume pot then tone pot. Still you are free to try out and see how it sound to you.
Big value tone pot doesnt have much effect when i was trying it out among various geetars i have. Using bigger value pot for volume has more effect.
The increase of resistance value, while making your guitar sounds brighter/glassy/piercing sounding what so ever, will see increase of noise level as well, in some instances.
heh, if you have google or read up before hand on the pot value and cap value generally used in guitar, you will know this..
single coil-250k
humbucker-500k
Tone cap- 0.022uf, 0.033uf, 0.047uf
treble bleed cap- small value of picofarad region, 180pf-470pf
anyway, those are easy info which can be gotten from google. If you are curious enough, you will continue to go search why the above values.
Hehe, have fun...