I normally use the "most appropriate" size, but this year I'm just using usize everywhere and it's a lot more fun. So far I haven't found any problem where it ran too slow and the problem was in my implementation and not my algorithm choice.

I usually use Longs by default too, but this year I mostly got by with Ints, so I've stuck to Ints for less memory (my solutions tend to be rather inefficient in that regard .-.)