250 lbs....You are looking at a xx-xxx large vest. the xxx large will probably be the most comfortable. I have a couple big friends and thats what they use. Oniell is the best but any Coast Guard approved vest should do the trick.
I think JetPilot runs small, definately try them on first. I was 5'10 and 205lbs last semester and bought the Jetpilot Stance L/XL and it was a little tight.
Once you hit a certain fat percentage the coast guard should just stamp your back "coastguard approved" I know that as of late i have been floating a lot higher in my non cga vest. Hmm i wonder why that is?