When we produce models in Gmax and Max, we use built in measuring tools in the software to accurately measure parts and things, from wingspan to wheels and things.
A good way to measure the plane is to make a block that is exactly the length of the said aircraft, export into FS as a scenery object, and part the plane beside it as a measure gauge and go to top down view and examine it. Side view will not work as you are dealing with 'perspective' so it (the block) will appear shorter from the side view.
As for scale in the world of FS, you will need to ask Aces that. But supposedly its all to scale. I have always thought the world was slightly out of scale, personally, in FS, but the planes and all should be 'exact'.