whats biggest joint of lamb I can use in a 3.5L slowcooker
You can use a slow cooker for as big a piece of meat as will fit in it and still allow the lid to close. There is no strict need for extra room in the crock.
That said, you will probably want some extra room for spices, stock, et cetera. But so long as you can physically fit all your ingredients, it'll all work.