All the revisions have the same form factor. Theere are also some motherboards (most) that will run x16 slots at x8, by cutting off rails. Because of the way this works, all pci express cards/slots are intercompatible, to the point where you could run an x16 2.1 card in an x1 1.0 slot if you wanted. (just an example, it will bottleneck) But some motherboard manufacturers close up the back of the slot, so cutting it might be needed to run a larger card in a smaller slot.
There are also mini pci express slots used in notebooks.