Best swimsuit for someone who wants to keep thighs a bit more hidden would be either a skirted suit/tankini, or a pair of longer board shorts. Here's a pretty good one I've found:
Unfortunately, it only goes up through an XL (size 16), so not for the more plus-sized, but if you're in that range it's supposedly a pretty "roomy" fit. Here's the text that goes with, and a link to it (if the link doesn't work, go to "swimwear', then 'board shorts and more', then 'real boardie'):
Real Boardie and Surf Skirt
Our friend Jen designs surf-inspired gear for real women, doing real stuff, with real bodies. Roomy flattering fit doesn’t bind and does fit... our body, our lifestyle and our life. Quick drying poly micro. Boardie: Velcro closure, 9" inseam. Skirt: Zip closure, 18˝" length on medium. Only at T9. Cement, Gypsy blue; S(4-6), M(8-10), L(12-14), XL(16)
Title 9 Sports Board Shorts
For tops, Title 9 carries a pretty decent selection of tankini tops and stuff, or just hunt around and find a place that has tankini tops, cause that's what works best with board shorts.