It comes in various sizes from about 2'x2' up to 4'x4'. I bought the 32"x44" size mostly or just what ever was on sale. you don't need thickness over .080" in most cases that's just adding expense for thickness you don't have to have.
You'll need a good sharp box knife and something to use as a straight edge (if you need it straight).
I usually plan out my sizes before buying then go to town to get everything at once.