When you are designing the gearbox, make sure you pay attention to the gears sizes and Pitch Circle Diameter (PCD) and you know what Module, or MOD, of gear you are using. (Google those terms of you need to, they are fairly easy to get your head around)

Linked below is a typical data sheet for gears, and it tells you the MOD, PCD and the Outer Diameter, as well as width and bore size.

http://www.hpcgears.com/pdf_c33/23.50-23.55.pdf

When I started designing my first gearbox it drove me crazy making sure I was using the PCD not the OD or that there was sufficient material to put in the bore I wanted to. Also, don't go with anything less than 12 teeth on a gear. While you can get 9T, 10T and 11T gears they are not considered good practice and tend to be expensive specialist parts depending on who you buy them from.

PS: You may have known all this, but given how much of a pain I found my first gear box, there is some advice from one gearbox noob to anther :-)