[EricDrost]

Not to be a downer but in most FRC applications a standard 6 wheel "drop-center" tank drive with grippy wheels like Colsons will bring you far better performance than a mecanum drive train.

However, if your team is dead-set on using an omnidirectional drive train, I cannot stress enough how important it is to build the system you want to use in the OFFSEASON. This way you can work out all the kinks and learn from your mistakes. Building an omnidirectional drive system for the first time during build season will bring unexpected challenges that will require much more time than you think in such a short 6 week build window.

As for wheel size... I'm by no means an expert but the better applications of mecanum I've seen tend to be using 6" wheels.