Help with plan Size
CBL gave you the educated way to do it and I agree with his method.
Here is a less scientific way of going about this task. Check out the specifications on a number of similar models including type, construction method, materials used, etc. that fly on the desired motor. You will find that a lot of the numbers will be similar, or at least close to each other. Guess-timate the wing area and use that number to find the desired span. Divide the new span by the old one to get the scale.
Remember, I didn't say this was the scientific way to approach this problem, but I have used it with (mixed) success in the past. And, the times it didn't work proved to be quite interesting.
I have to add that I only used this method when I could afford to throw away the time and money involved in the project.