I made it as long as the car, from under the core support to under the rear cross member. Think is was 14 or 15 feet.
I feel you can't have to many support points when working alone, makes it much easier to get things right the first time. Making the dam support legs is a pain, but you only gotta make them as you need them.
The floor in my shop is almost flat, but not perfect it can vary 1/16th" =/- over the length of a 6 ft level. On the bottom of each leg there is a 1/2" hex nut welded to the back side of the pad.
If I want to get it perfectly level I can just put some bolts in .
Level is not totally important if you are building from a datum line, as the line is only a reference and doesn't know up from down. When I get to the point of being critical I will set a laser line up for a datum and then fine tune the table to it. then the car,lol.
I put a 3/4" hole in each leg, I have some 12" diameter rubber wheels with 3/4 roller brgs.
I can slip a 3/4 bolt through each wheel and table leg and be able to roll the table around.
It raises the table 2" off the floor when on wheels.
Isnt real good for tight turns! but it rolls in and out of the shop and onto my trailer quite nicely.