The tank itself is 12.25" from front to back (including glass)
The glass is 3/16" thick on each side, combining front to back is 6/16" = 3/8" (0.375") total glass thickness
12.25 - 0.375 = 11.875"
You'll want some wiggle room to put silicone between the 2 glass, so I suggest 11.75"
Since its not really holding water, but just getting rid of microbubbles, a thin glass is actually suffice. 1/8" or thicker, thicker is generally more stronger to withstand minor bumps. I personally used 3/16"
Height is usually 6-8" water depth for sumps, so the baffles itself should be that or less