Without having the plates to actually test it myself - I would think that it would have to be a 14" square cake on a 16" square plate - MAYBE you could do a 12" square on a 14" plate. The picture you've shown is definitely on a 16" round plate and the fountain has plenty of room. Since a square offers even more space - I thought the 14" plate might be possible. I think the actual fountain is about 12" wide?
I've probably just confused you more so hopefully someone else has some exact measurements to either confirm what I'm saying or that ability to set us both straight! Good Luck!
Here's a pic of a square-over-a-fountain. The bottom tier is 12". A 12" plate won't work (read my narrative under the pic). http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1155458
Per your other thread, is the bride still looking for only 150-ish guests?
Yes she is still expecting 150- 175 guests. Plus I am also making a groom's cake.
Thank you so much that is the info I needed. So what size square plate would work with the fountain? A 14 inch?