Elad:
"If it looks like it will fly, it probably will." Is it almost that simple?
Just about.
If you find a good design that's smaller than you want, almost without exception it will still fly well when made larger. The reverse is not true. Save the smaller versions for later when you have more experience.
If you are scaling a wooden plane up to 2x don't double the wood sizes, go to 1.25x or in some places 1.5x, but on the skins you might well stay with the same thickness. This is where you can run into trouble with your first few scratches, not knowing how to make it strong without making it heavy at the same time.
When we enlarged the Duellist 2/40 and kitted the Duellist 2/60 we went from a little over five feet span to more than seven feet with almost no increase in weight.
Enlarging a SPAD is just as easy, and a lot cheaper. You might need an additional yardstick spar, just look at others in the size range you want. There are several sizes of the "QUHOR" to choose from, for example.
Usually I discourage a first time scratch builder, but the wannabe scratch builder often starts by asking what glue, what wood, how big, it becomes obvious he has not had the experience needed. But if you start with an existing design and scale it you might consider it to be a scratch, but it's more a plan build, and you have the guide to lead you.
Would advise a couple of kits to get the idea of material sizes, so forth, before attempting to scale one, though.
Bill.