Now for the easy way
Now that you know the proper way heres the easy way to do it. scropunge the net to find similar planes what their ws is. ie same thing the guys above are telling you to get your desired final outcome. then take plans to kinkos and tell them the wingspan on the plane you have and the desired wingspan they will figure out the resize percentage slapo your plans in their machine and print you out a set of plans at the proper scale. remember you have to get the plans printed somewhere why not let someone else do all the calculations.
Joe
PS it would also help greatly if you tell us what plane you are wanting to do? some people may have already done them saving you headaches