Balsa comes in 4 ft. lengths. That gives you 8 ft (plus tips) with only a center dihedral joint and that's about as big as sheeted foam wings go. Center dihedral joints are universal, and on sheeted foam wings they are always glassed on the outside of the sheeting. The glassing is then hidden by the fuselage.

Jim

PS: One layer of 6oz glass is more than adequate for your airplane.