To scale it down on a copier I think use 72%. That should be close enough. That should give you about half the wing area.
What engine to use? That's really a matter of personal preference. If you don't care about a throttle use the TD's! They're pretty nice engines. A common tank with a T is a good idea for getting them both to shut off at the same time.(or close anyway)
Why only half the wing area? I'm shooting in the dark without more info on the airplane. How much wing area does it have with .15's? Or another question you may have is why not 1/3 the wing area? Well, the short answer is that your TD's will haul ALOT of balsa. Remember your losing weight with the smaller engines and servos. An example is a delta that I have that's got 360 sq in of area. It flies and makes full use of a bigmig .061. It also flies with a .15 on the front and bigger servos, battery, etc. The delta flies well in both configurations. Faster with the .15, better landings with the .061. Could it fly with an .049 or a .20? Sure! I hope this helps a little. Finding your All-up-weight and wing area are the two main things you need to know. After that you can figure your wing loading and predict flight characteristics. One other thing that's important is the size of the cowls compared to the prop. It will look silly and out-of-scale with a 5" cowl and a 5" prop. (well,.....it will!)