I would choose a rectangular w/ rounded corners/bottoms. The ratio of depth:length:width would be 1:2:3. Ideally, I would want it to be 6'd x 12'l x 24'w.
I like it because it would hold the maximum water for the allowed space (you could always go deeper). Those dimensions would keep the plumbing fairly simple. You would only need two bottom drains and one skimmer. No need to reversable TPRs since the current flows in both directions.
Here is something I drew up a while ago. This show a basic semi-raised pond layout.
-Dan
