Best educated guess of this tank size...

[doomiedee]

I've owned 6 and they've all had them.

+1 every 55 I've owned and seen had center braces. Unless its one of those old ones with thicker glass.

 

[zukiGS500]

Looks like 12 inch wide tank, and assuming it's on a 2x4 stand. It roughly 7 boards high, 7x3.5 is 24.5, I would guess a 48x12x24.

Looks like a 60 gallon.