Indeed, grab a VIP ticket so you can have a tour of the pits. It is worth the money. For what it is worth, my first iteration of my featherweight was 36cmx36cmx22cm (length x width x height). With a robot of that sort of dimension, it is worth laying out the components in cardboard, as you say. It is also worth noting that unless properly planned out, wiring is a pain in a small robot like this.

However, I am going to second what Mark said. Spinners are dangerous and having had done a spinner as my first robot, even as a low power variety, I've had spinner arms/tips fly off at speed, not a pleasant experience to put it mildly. You can't really test a spinner either in anywhere else other than an arena either simply because of the dangers of them exploding. There is also the problem of the live events (thinking of Robots Live! here) not always accepting spinners because of the arena not having sufficient protection for them. Extreme Robots can handle spinners, as can RoboChallenge (who do the FW Championship at the moment) but Robots Live! can't so it does limit the number of events you can go to.

I was pretty sure I was being dim when I suggested adding a belt/chain when there's already a gearbox but I wasn't sure.
I mean, you could use a 1:1 belt or a simple 1 stage 1:1 gearbox but as I noted, it adds another failure point and more weight/complexity. With that said, You could offset the wheels to allow for this.