Most of the dimensions are below. The width between the rails is 8 inches which is just perfect for the rear tire, you may want a half inch or so more for clearance, I just wanted to keep it as small as possible
It's made with two 2x4s and and 5/8ths OSB ply. Some people use just a 2x12 but it flexes with a bike on it. With 2x4s and OSB (glued and screwed) it might flex like an eighth inch, so it's pretty rigid.
To motorize it, I made a ring on the bottom with the OSB so the garage door chain (with a few screws to keep from spinning free) would have something to pull against. Make sure that the ring is perfectly concentric to the axis of rotation or the chain will bind, I did this by using a router mounted to piece of scrap wood that had one screw acting as a pivot point in the exact middle of the bearing mounting bolt holes. Then I used some scrap 1/4 inch plexiglass to make a "lip" to help the chain stay on the ring.
The garage door opener is mounted to the floor with concrete anchors. I also made some chain guides with OSB and some metal flashing so the chain stays under the turntable unless rotating, and used some of the 1/4 plexi to keep the chain off the floor and better aligned.