for the large rays consider a 600 gallon to be the minimum size... that will hold 3 of them pretty nice... ponds are ideal for rays as size to price ratio is better...
they can out grow it marbled and regular motoros get the same size.. they can get 24" across the disk and a standard 300 is 8'x2.5'x2'... a I.. will give you many years before they could outgrow it at that time a pond would be your best alternative for them best bang for the buck....