RE: Spirit elite 2.5m mod
You can probably stretch the wing to 2.5 meters but with the stock tail and fuselage that's about as far as I would want to go with it. Beyond that the tail and moment arms will be too short to dampen the wing's forces in an efficient manner. If you want a larger model, and larger does indeed fly better, then go for a properly designed larger model in the first place. From there just pick the model that has the style of flying you want.
As for the controls I'm not sure what you mean by "full span". With gliders the ailerons are on the outer portion of the wings with flaps on the inner portions. So control surfaces already take up the whole span. If you're thinking flaperons then I can say with conviction "FORGET IT! THEY DON'T WORK". Tried it twice with terrible results.