250k is standard with single coil equipped guitars, 500k with humbucker equipped. 500k potentiometers preserve more treble than 250k.
A/B is the taper: A is logarithmic - regularly used for volume, B is liner - regularly used for tone.
Dimensions:
18mm shaft length, 24mm base width.
Supplied with nut and washer.
See our printable PDF to check for size - https://drive.google.com/open?id=1OhI9VYKwJbtYhd_oxg42VVAbc7LUGVut