The answer depends on what pattern you are planning to make. I would suggest that you find a pattern book that you like and go from teh directions in there for your first large quilt. Check the library, they usually have a ton of quilt books. Good luck!
And if you know what you want to make, post it here and people will probably be better able to help. Also, if you search atthe top of the page for "bed sizes" there are resources to figure out how big your quilt should be to cover a twin size bed.