i believe their 6x5x24", but they slope in towards the bottom. not shure on the actual usable floor space. im just trying to get a timeline on how long i could use one if i go this route.
I used a 150 (half the size), and I would say it would easily house them for 2 years. A 300 is pretty wide, even if it is a bit sloped. It wouldn't be for life, but they could definitely get a decent size. All in all, the 300 gallon rubbermaid is the best bang for your buck (besides cutting a giant plastic water retention tank in half), and I can see no better alternative if you're not worried about having a viewing window.
To put it simply, yes, it would accommodate a couple of very comfortable motoros easily for a couple of years. Makes an excellent growout tank.