The Super Off Road games are multiplayer and are a lot of fun with 2 players, haven't tried them in 5 but 4.5 worked great.

As for the hand wheel I would go 4" max, I was thinking the bigger the better but these hand wheels are lower than the height of the joystick by 2" (I'm using bat tops but stock ball tops are still 3/4" taller roughly, so a 5" would just barely clear the joysticks and your fingers wouldn't clear the sticks while playing. You also wouldn't want to put too much torque on the spinners by using too big a wheel but the 4" seem pretty safe.

These are what I use: https://www.amazon.com/gp/product/B073CCLGCC/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I earlier used a couple of these https://www.kidswheels.com/index.php?option=com_mijoshop&Itemid=606&route=product/product&product_id=859379&search=steer&page=4

and was able to attach them with some pvc pipe shims. It was pretty slick and the pvc just snapped onto the base of these wheels and then snapped onto the base spinner and was sturdy enough to play pretty well. The wheels are closer to the size of 80's arcade wheels and they sat above the joysticks and could just be pulled off with a little force making swapping them out super quick but the 4" hand wheel is probably the best solution for this. I leave them on and it seems like you could play spinner games fine with them and still be able to play everything else with the joysticks without having to remove them. They really do look like they were made for the ALU.