I am a size 7 jeans-wearer, with a 36" hip and based on Hanes' sizing chart, I should be wearing a size 6 (Medium). To be on the safe side though, I ordered size 5 (Small).... and they're still too big :S. The undies are wearable, but as far as I can see Hanes sizes are waaay off.

My advice - if you're on the smaller side, go with a size 4 (XSmall).