Okay, now we all know how large Turpin's regular TCS sporrans are, right?
For comparison, TCSs #28 and #55:


Frank with his flap open:

I know it's kind of hard to read the tape measure, but the sporran body is 10" x 10". You can see the nice thick leather Turpin used for the flap backing and the sporran body. Also, notice how the fur on the front blends into the separate strips on the gusset with only a slight indentation, and no gaps. And the fur extends all the way up to the trim around the top, and over the edge of the flap, so no leather is noticeable on the flap when it's closed.

Shot of the inside:

Not the best shot, I'll admit - it's hard holding the sporran upright and open with one hand, and hold the tape measure in place with the same hand, while taking a picture.
You can see the standard phone pocket (with business cards) and can just make out the key holder. The pen holder next the phone pocket blends in to the back of the sporran. If you look behind the cell pocket, you can see where the flap slips in and snaps in place - yes, the flap is convertible. It is a TCS after all.
The tape reads 4" (or it would if my fingers weren't in the way).
And though you can't quite see it from this angle, there is no snap or catch for the flap. Frank's head is heavy enough to stay down while being worn.