If you are afraid of starting a larger one all at once, work you way up to it. Try a lap sized one or a twin and then larger, a full sized, ect. It is all a matter of getting used to making things in a big way.
There is always using a pattern tat works up into say four or five small squares and then is sewn together. That is a little easier to manage and will get you the results of a large quilt at the end.
Check out a couple books on quilt as you go. It might just give you the answer to your problem.