I can tell you that bigger is better for a baja shelf. We made ours 18" deep, and it is about 18' by about 10'-12' (big pool, too). I do not regret one inch of the size. We love the deeper shelf for getting in slowly when the water is cool, for having a splashy place to play for the just-learning swimmers (when they were much younger), and for a place to actually sit in the water. Finally, our non-swimming dogs finally began to venture in the pool (which we enjoyed). They go in every day that it's hot, and sometimes when it isn't.

Ours is 10' L x 7'6" D, in addition to the 15' x 30' pool area. Worked great when we had a 2 and 4 year old visit.