How do you know how long you need to draw the vectors? Like if the gravity vector line is longer than the normal force, or if the friction line is longer than the applied force or whatever..?

Answer 1

unless you're trying to solve a problem graphically, drawing to scale is not important. Just label the vectors appropriately; the magnitudes will give an approximation of the lengths. The calculations don't depend on how accurately you draw the picture.

Naturally, if one vector has magnitude 5 and another has magnitude 10, I'd expect the 10 vector to be drawn about twice as long as the 5 vector.