Official LEGO PDFs often have 1:1 scale drawings of axles and beams. If you are unsure of a part's length, hold the physical piece up to your screen (though keep in mind screen zoom levels may vary).
building instructions PDF is the first step toward bringing your creations to life. Where to Download Official Instructions lego mindstorms nxt 2.0 building instructions pdf
Do not print the entire 100-page PDF. Instead: Official LEGO PDFs often have 1:1 scale drawings