As a not so small active person, another source for plus size swim wear is
www.junonia.com This company does some great stuff that is modest but most importantly one can actually move in. The biggest down side is the price, but they do have sales/on-line coupons.
I also agree that Lands End is wonderful as well. If you have questions about fit, there staff at their 1-800 does know there stuff.
Good luck to all on the quest to find the perfect swimsuit.