I flew SW this weekend to MCO, from Tampa.  They were VERY picky on carryons.  I had a small suitcase and a small sling bag (w/ a dvd player) and my small wallet size purse on a strap.  They told me one of them had to be checked or I had to put one inside another.  On the way back I had a tote bag, my purse and 'to go' food as we were going to be flying over 8 hours with stops & changes and no time to eat.  I was told the purse or the food had to go.  Fortunately I stuffed the purse in the food bag and they accepted it.  These flights were not overly full.  So, if you carry a purse, you can have 1 other 'thing' regardless of size.  DD had a small backpack purse, a book and her CD case and although they would all fit at least a couple of times over under a seat, they told here she couldn't have 3 things.  The book hanging 2/3 the way out of the purse was fine, tho.  Go figure!