Overweight people come in many shapes, of course. So it may not be possible to predict exactly what will fit best, but here are some suggestions:
VW New Beetle--it has a lot of space in the front, and can accommodate heavy and tall people surprisingly well. The back seat is useless, though.
Toyota Avalon. I have a friend who is overweight and mobility impaired, as well as very tall. Her Avalon is a very good fit for her.
Hyundai Sonata. The range of seat adjustment in the Sonata is very wide, so it's probably easier to get comfortable in one of these than in most midsize sedans.
Hyundai Elantra. The roomiest compact car on the market, by a considerable margin.
You might also try to find a recent Ford Crown Victoria. It has a front bench seat, which many larger people find more comfortable. Not sure about the legroom though.
One other possibility might be a crossover or SUV of some sort. In particular, the Buick Enclave, a big car-based SUV/crossover, is very spacious and comfortable (but expensive).
Good luck in your search. Sometimes the only way to figure out what works best is to try a bunch of different models on, perhaps at a local auto show where there is no pressure to purchase.