I did some changes on S3v3rino board, and got this:
I need to make more adjusts, but I think it's the smaller board size without serial components (looking it better, I can make it a bit smaller), but Arduino fully compatible.
The sizes:
66x84mm original SSS (version2)
66x90mm S3v3rino
57x76mm TTL version
If you move C5 and C8 closer to the border (near the regulator), you will be able to lower a little bit more the cost, by using standard height capacitors. Everything in the SHIELD area, needs to be low profile (shorter more expensive components).