Materials of the electrode: gold, Teflon, brass etc.
The gold working electrode (2mm) is as follows:

The most commonly used is 2mm. Gold working electrode of other diameter are also available.
The diameter of the Teflon sleeve is changed with the diameter of the gold.
Diameter:
Gold | Teflon sleeve |
2mm | 6mm |
3mm | 6mm |
4mm | 8mm |
5mm | 10mm |
6mm | 12mm |