There is a nice composite materials handbook, "R&G Handbuch Edition 06/2009, Composite Materials Handbook" (both in english and germany) available free on the network at the following address:
There starting page 80 you can find instructions how to estimate the airplane structures roughly:
Wing Spar Cutaway
"The simpliied example given shows how preliminary dimensions can be roughly calculated for fibre composite components. The values are based on material characteristics encountered in glider construction. Exact dimensioning may necessitate determining the material characteristics from case to case, which vary depending on the manufacturing method."