start by deciding what feels big (or at least bigger than how you normally work), start from there and work your way up.
working large requires the space to work large! before deciding how big you want to go you will need to figure out where you will work. the floor is my favorite place to work large. i like to spread out my materials and crawl around my surface (though i must warn that it's hard on your back and knees).
the wall is another great place for working large- if you have wall to spare, you can staple your canvas, fabric or paper to it and create standing up.
an easel 