That should be a fairly easy frame to design. Length doesn't really matter, you just keep adding bents, widths are another story. You can only go so far before you have to put a post, or a complicated truss. 16-18 feet is pretty easy, especially if you don't need any load bearing for a loft.
What type of siding, horizontal or vertical? That makes a difference, more studs for horizontal, or add girts for vertical. Heated?