Are all forks the same size i.e 1 1/8th inch?
No there are a variety of standards. Off the top of my head;


1" threaded (not common on new bikes)
1 1/8th" threaded (not common on new bikes)
1 1/8th" threadless
1.5" threadless
1.5" - 1 1/8th" tapered
Cannondale headshock size (I think this is interchangeable with 1.5" though)


Typically you can only replace like for like. There are a few exceptions, e.g. 


1 1/8th threaded/threadless designs can be used as replacements if the headset and stem are changed as well.
1 1/8th threadless can be used in tapered headtubes by fitting a step-up spacer to the fork crown.


The threaded designs tend to be limited to older bikes or really cheap new bikes.

What bike (make/model/year) do you have, it should be easy enough to work out from that what standard it is fitted with. However if you are unsure any good shop should be able to advise.