If you check in online 24hr before the flight you can access any seat including exit rows (that may be your best bet if you are a big guy). You can also call CO - sometimes the phone rep will move you, sometimes they wont. Oh - if you do get the exit row on a 737 or 757 be sure to grab the second one over the first one (e.g. if row 13 & 14 are exit rows go for row 14) as you will get the extra leg room and a seat that reclines.

Best of luck,
J.Edward