250k is standard with single coil equipped guitars, 500k with humbucker equipped. 500k potentiometers retain 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, 45mm height
Supplied with nut and washer.
See our printable PDF to check for size - https://drive.google.com/open?id=1B7IGtpibLByCTtDKdYwGNfWd6SmfG-rM