Id suggest showing them an example of an entry and ask them if it will work. If it is indeed setup as you say, then only 3 out of the entries so far will work.In terms of the size, with ours the center sections is stretched a little. dont ask me, talk to my designers. they gave me the sizes...
If its imperative that it stretches, than the middle area needs to be designed to accommodate that feature. Basically rendering all but 3 submissions useless.