6"x6" would work with out bowing or putting in vertical posts, if you interlocked the corners correctly. Just built a pond that is around 1.5 to 2 ft above ground and 18" below ground and 7'w x 13' long with 6x6's above ground. My biggest concern with wood is where it contacts the ground. If you don't get wood that is treated for ground contact, it will rot out in a few years no matter how it is built. Read the tag on the wood to see what treatment it is rated for.