A 450 is probably the best choice for sure, if you have one. But if you already have a 250F, go for it. And you don't have to do a big bore on it.
A friend of mines kid took one of my old TS 137 with a 2.5 track on it and stuck it on his 250F. Everyone told him it wouldn't work well cuz it was gutless and that it would immediately fly apart, but it worked awesome for his first snowbike season. Obviously he couldn"t climb the same lines that our bigger bikes could, but he would just find another way. He could still sidehill everything and drop everything. Being a fearless 20yr old that was born in the mountains helped a bit too, I guess. The funny thing is is he sold it and bought a 450 and immediately blew it up. Go figure.