Ken, I think your on the right track with the weight and engine size like your last post. The added wing area will just give you lighter wing loading and should make for a better flying plane. I don't think the weight of the covering is going to make that much difference in the final weight of your plane, some...but not much. IMHO, there isn't anything wrong with using a bigger engine that you need, that's what the throttle's for, and it never hurts to have a little more power.
Suptter