I don't know exactly how large the boards are, but, judging from the dimenstions of the computer itself, the board is less than 15"x17.1" for slot-loaders. I don't know if the different versions of the iMacs have different sized boards, but it might help to know which iMac you have. (You can get help with that on
http://www.apple-history.com, if you don't know exactly.)