fundamental rule = pictures/blueprints...
Example from my (still unfinished project) KTM 640 Supermoto. Over 500MB pictures from different angles (avrider forum is strong with details) some original pictures from manufacturer (manuals are very useful, you get exact parameters/dimensions/radius brakes/wheels, wheelbase etc...)
If you didn´t find these parameters, you can go for real dimensions from other side. If you know just one dimension (brake radius, wheel dimension, fork length, simply use it...And then "adapt" your blueprint to model created in real dimensions. Ofcourse, it´s not super exact, but enough...
Also can be used "markers" (you can see it in one of attached pictures). Simply mark key axis or screw positions or some important points on skeleton etc...