Fourtrax 250 weight limit
 

So I'm trying to get a fourtrax 250 but I'm a real fat man 350+ but that's bout all I can get now do you think it will push me at a decent speed how fast will it be? At this weight I'm aware it won't be as fast as a 500+cc atv

Your pretty close to the weight limit but you won't brake anything unless you start doing bigger jumps.
The speed shouldn't be a big issue. If you where actually serious of going super fast you'd be getting a bigger quad