Nice tank! I would guess something like a 45 or 50g?
Does it have a sticker on the base panel stating a brand or date?
Due to its shape, and unless you already have it, making or getting a lid for it is a bit of a pain.
I recently got a free pentagonal 44g and resealed it. It is now being tested for leaks, since last night. So far so good. It did come with a lid (very narrow opening, barely my arm), and the matching stand seems very similar to yours.