[DampRobot]

One thing I recommend experimenting with is how easily you can get out of T Bone pins with your drive base. If you're planning on putting up a significant number of points next year, you will get T boned more than you like, and you likely will wish you designed your drive base to make it hard to T bone pin you. Think about what frame dimensions will give you the most turning moment against a pin, and what "add ins" like drop down casters, omnis, etc, can help you get out of pins. Oh yeah, and it's a ton of fun practicing pinning with another robot, so I encourage you to test it after you built the base. You'll get ideas for improvement too.

Test out a bunch of different wheels (VersaWheels, AM stuff, different treads omnis, etc) to see what you like best in terms of traction and turning. With a WCD, it's super easy to change out wheels, and you'll be glad you bought the wheels (even if you don't like them all that much) because they'll be a part of your inventory for next year.

Make your bumper mounts rock solid. So many people (including me) mess this up the first time around with WCDs. Design the side bumper mounts as if the robot will be slammed on the side with sledgehammers during an earthquake. Trust me, this is one of the few things on your robot you will not regret overdesigning. And then test the heck out of them by going to offseasons or by having another robot slam the bejesus out of the bumpers for days in end.

If you can, put some kind of manipulator on top of the drive base you're building and go to an offseason or three. Planning on going to a competition will give you a big motivation to finish, and will be a ton of fun once you get there. Sometimes, grand summer plans can fall by the wayside because there wasn't enough motivation to get them done. Deadlines (and looking forward to offseasons) helps with that.

Oh yeah, actual dimensions. I'd go 28x28 and see how you like it, but it really doesn't matter too much either way.