From the apple tech specs page for the TB 27":
Height: 19.35 inches (49.1 cm)
Width: 25.7 inches (65 cm)
Depth (with stand): 8.15 inches (20.7 cm)
Weight: 23.5 pounds (10.8 kg)
I measured my 27"s width (mini-displayport model but I don't thing that matters) and measured the depth of just the monitor and it's approximately 1.25". I say approximately because the back is slightly curved toward toward the center so I had to just eyeball it with a ruler along the edge. I'm mentioning this because of what you said you intend to do so the published depth with its stand is a useless number.
Note, even if the monitor depth itself is about 1.25" the power cable that plugs into it in the back has a stiff rubber color so I would allocate at least 2" for the depth to accommodate it.
Isn't there a local store where you can see one of these monitors?
Update: Sorry I don't have specs on the rounded corners. But FWIW the curve looks about the same circumference as a quarter.