Is there any size limitation for this contest, some one said it had to stay in the 18" cube the whole contest, can’t find it anywhere?

Yes there is. Take a look at the rules here.
Specifically you should look at rules:
<R4> on page 25
<SG3> on page 13
<SG10> on page 15
as well as the relevant terms in the Game Definitions section starting on page 5.

After reading that come back and tell us what you think the specific answer is to your question.