[hrench]

When I've designed gears for machines, I've followed the criteria from 'Design of Machine Elements' by Spotts; there are two criteria that need to be worked out before you decide on material, diameters and facewidth.

First you need to work out the bending capacity of the teeth. without copying the whole text and the equations, I'll say that the bending is rarely the controlling failure.
Second, you work out the 'tooth capacity in contact stress' This involves not only geometry but also the moduli of elasticity of the sets of gears in the train. If you've ever taken apart commercial gear trains, you'll often see that the output gears may be steel but the earlier gears in the train may be brass or even plastic. The equations show you that different moduli make for better wear /stress capacity.
But you can get more stress capacity by increasing the face-width, so sometimes this is a substitute for using varied materials.

So the point is, if you want to 'design' gears, I recommend actually running through the equations and see where you are. A 'standard' face width may be fine, but just assuming probably isn't. if you need these equations, I can get them for you.