I have a Sram 10 speed rear mech I’m planning to use but I don’t know what length the cage is. Any technical people out there know the measurements of a medium and a long cage mech centre to centre on the jockey wheels?

put a picture up… they’re REALLY quite obvious.

as above, it is dead obvious when looking at them. If you can’t be bothered with putting a pic up, have a look at short/med/long mechs in crc and then you’ll see how obvious it is.