I would like to take my 1/7th scale plans of a MIG-3 and enlarge(Xerox) them to 1/3rd scale. When I do this it will grossly over space the ribs not to mention enlarging all the balsa wood sizes. So how do I get the proper thickness of wood and spacing on all ribs and formers?

TLAR Not really joking but that looks about right works pretty good with common sense. Just look at the size of wood used on ribs in a 40% plane and a 1/4 or 1/3 scale. 1/8 is about as thick as it ever gets. I could look it up in one of my old scale building books to see if there is a real formula but TLAR has worked for me pretty well. If the plane gets fully sheeted it doesn't mater a lot. Most of the strength is derived from the sheeting.