There are indeed size restrictions. However, this year was my first WF and I didn't enter anything, so really don't remember what they are. From what I can recall of this years WF, they're fairly liberal.
For instance, several groups had entries that exceeded the single entrant size restrictions. However, you are allowed to combine entries, thus somewhat "bypassing" the restrictions. This means that some groups are able to have huge diorama's. A group calling itself the "TechMages" (mostly from the Chicago area) made what was called "J-57 - Jeffries Station", a 1/1000 scale Star Trek space station. The station itself was easily 4' tall and had a fairly large area of table space taken up.
Another group of guys had a Mech battle display that took up probably 4' across by 3' or so "deep" and perhaps 12" high. It was most impressive.
I checked http://www.wonderfest.com/ <---The Official WonderFest website. Took me a bit, but I finally found the contest rules section in the Online Flyers sub-section. Click here to go to the --->: http://www.wonderfest.com/contest.html - Model Contest Page.
HTH,