Steve:

If you increase the size of the vertical fin that will make the rudder effectively smaller. I'd leave it alone, enlarge the rudder only.

You are going to do the engine runi-n on a stand, aren't you? For me two ounce tanks would probably be fine, but if you want 15 minute flights you'll want bigger tanks. You can check fuel consumption as you do the run-in, and size your tanks accordingly.

Bill.