obviously eye to eye length is easy, how do you measure the stroke length though? Is it the exposed piston length?

Probably but not necessarily. Take the spring off or remove all the air – extend it – measure eye to eye then compress it ( again making sure no air in it if its an air shock) and measure again

top pic shows how:

Thats brilliant, Thanks

You’ll find it hard to get the correct reading, as mm can make all the difference. Best way is to read it off the side of the shock, or to ask the manufacturer.