On my team, we had the mock bot (known as the Mauck bot, because our teacher Mr. Mauck used it for programming class) to teach the programmers and freshmen. It was a couple of feet long and not too wide (I think the designers tried for the golden ratio) and only as tall as it needed to be to fit all of the components. It was made out of our more expendable electronics and our durable wheels to ensure that minimal damage was caused by the occasional fire. The crio was borrowed from our competition bot, and we switched it back and forth as needed. We had it on four wheels, with old cim motors on the back two and space for them on the front two.
In my experience, if the robot is for the benefit of the team, it is best to use components that you wouldn’t necessarily miss. If it is for public demonstrations, durability and consistency is key as opposed to a ton of action. People will be entertained by a robot that works at all, so it’s best to make sure that it works every time.