Rear mech long/short/medium cage

I want to replace my rear mech as I think from troubleshooting its the reason for poor shifting performance. They SLX/XT ones tend to come in short/medium/long cage types.

I was after some advice on what this is (i presume the distance between the jockey wheels) and also how to choose the correct length for my bike (Spicy 316).

Thanks in advance. JP