I currently have a set of wheels with 255's on the rear but they need replacing but how do i find out what size the wheels are so i can see if i can fit bigger tyres??

the size is either on the back of the wheel (j size) or you'll have to take the tyre off and physically measure it, dont measure it from outside edge to outsie edge, you'll need to measure it from inside lip to inside lip, with them tyres on your probably lookin at 8.5j

I'll have a look on them to see if there is anything stamped on the back of the wheel.

there usualy is or you could always take them to a wheel balancing place and get it put on the balancer, that would tell you the size.