It's best to use the largest wheels that will fit in your hatch, I use golf pull cart wheels, there light and they float. Mine builts out of 1/4" aluminum with an aluminum axle. Looks like an flatend inverted "A" with the axle mounted thru an AL. box at the top, Al. rails are mounted across the legs of the "A" and hold the boat. It's a copy of a $115 version I've seen around (The Ouzel Cart). It's best to have the wheels centered of the cockpit if you plan on hauling it any distance.
We have to go from parking lots to the ferry and than about a 1/2 mile tothe launch when tripping in the Puget Sound. The big wheels help get up curbs and off the ferry. The centered wheels help the arms.