Definitely go with small. I am 6 foot with a 30-33 inch (fatness varies) inseam and wear a medium in their bib-shorts, which fits me great. You may even want an XS if they make one. See if you can try some on first.
I own two pair of their Laser shorts, which have the same chamois and fit as the top of the line Formas, but are cheaper, probably due to less fancy construction. I also have a bunch of Castelli stuff and like these a lot better. They stay put and don't have those stupid scorpion stickers everywhere...