I just bought the Oasis maxi dress from last week's Swears By e-mail and lord above it is fabulous! I was very skeptical, especially as I tried the large first and it looked horrible and highlighted all my lumpy bits, but the medium was perfect.
I'm 5'2" and 10/12 and I wouldn't attempt to pull off a maxi because they only come up to the top of your boobs so your clothing seems to start really low down. FAR FAR better to wear a dress or skirt and top that covers your shoulders and goes to your knees. That will seem 'longer'. Honestly.