Clutch slave size?

[Mr Gazza]

Please could anyone tell me the diameter of a standard clutch slave piston?
I'm after the size of the external type that bolts on the left side alternator cover.

I'm asking for a very good friend who is looking to make his wife's M600 clutch lever a lot lighter to help with her arthritis.
The 600 is an early one with the internal clutch side slave.
So being a very competent engineer, he is looking into boring the cylinder out and making another piston.
He did a series of experimental slaves for my Commando and they all worked well on a test rig, so he knows what he's doing.

If I knew what the standard size for the other side is I could compare that to the aftermarket ones to give him a guide size to aim for. Standard on the internal slaves is 26mm. I've got a feeling the external ones are too?

So far I have 29mm for Oberon and 27-27.5-28.5 and 30.5 for Elovuzione slaves.
If anyone knows what Factory Racing and others are it could be useful.

Thanks in advance.