If your hip measurement does not exceed the XS guideline in the manufacturer's size chart, I would go with XS. My hips are 35", and their size chart says up to 36" for a size S, which is just right for me in Assos. Their cut is snug-fitting, so when they give a measurement there won't be extra fabric hanging around.

Just as a side-line to this sizing-up debate, I am a consistent small across the board. I wear small in PI, Sugoi, Shebeest, Castelli, Descente, Craft, Hincapie and Louis Garneau. I have not been able to get enough of me into an XS to say I tried it on, it's always been too tight.

There are a lot of fit issues that don't affect you if you are just looking at a pair of shorts. You don't have to worry about tight across the chest, or leg-length. The waist is very forgiving & comfortable in the Assos short. It is a single panel of lycra that stretches like a bathing suit.

That said, I would still probably order my first Assos garment from somebody state-side with a good return policy, either Colorado Cyclist or Lick Bike in Chicago. It is worth the extra $$ to have the peace of mind that you can return if it's a complete bomb on sizing for you. Once you know your Assos size you can order from anywhere you find a good deal. But for your first shot, the savings wouldn't be worth the worry & aggravation for me.