The key steel is a standard 3mm key but as my kit is a DIY kit and can be fitted to many different types of motors shaft  so it is large enough to do so.
This does mean it has to be sanded down to fit tightly on some motor shafts,this is the best way as you need a really tight fit.
Basically just get some fine sand paper and lay it on a flat bench or surface then holding the key steel in your hand just sand each side until it fits on the shaft and in the motor pulley.
It wont take much to get it to fit – probably 0.1mm at the most..
The length may also need trimming with either a file or a hacksaw and file.
Take care with this- you can alway remove more material but never add more and it wants to be a nice tight fit..not loose..or you will lose the key way on the street!!So sand – then check,Sand then check….
If you have a set of needle files (cheap on ebay) sometimes a small filing of the inside of the pulley helps also.