Here is the main PSU installation point with the PSU window installed. There's a single screw (circled) to remove the PSU window. Be careful with it - it's acrylic and scratches quite easily.
Replacements windows are listed on Corsair.com where they have a clear window and a tinted window. However, at the time of this post, neither one of them is actually available.
The acrylic window itself is 171.5 mm (6 3/4") x 60.3mm (2 3/8") and 4mm thick.
The opening for the PSU window in the case is slightly smaller at 150.8mm (5 15/16") x 58.9mm (2 5/16 inches).
This whole area really just begs for someone super-creative to do something super-cool. There are all kinds of wild things that you could potentially do here, from small monitor screens to custom artwork and/or LEDs.