RE: enlarging plans?
For spars and similar items you need to use whatever is normally used for that size and type of model. You will just need to use the models shape and design a new structure appropriate to the type of model. Its not a question of just enlarging the plans when its such a big increase as 50 percent. A 60 inch biplane is an awful lot bigger & chunkier than a 40 inch one!
It's better to buy a plan already designed for the size you want.