Try a promini -I think they are just 0.65" to 0.7" wide.
If you need a bunch of them, then perhaps roll your own and make a promini like design, but thinner and longer, with the corners of the chip closer to the sides like I did on the Leonardino here, which uses a bigger package, and fits on a <25mm wide board.
Cross Roads Electronics scroll down ~1/3 of the page
Or maybe do the same with a DIP, like the Solarbotics Ardweeny
Use surface mount parts, skip the header, plug the part on a breadboard for programming.