Hi Terry The one time I poured a large monolith like you are describing, I got very sore arms. I used two very straight 16 ft 2X6's with 1X4's screwed to each side to keep it from sagging when I screeded to a 2X6 center board on a 28 foot wide slab. I did both sides of the center board at once, doing one side for a bit then moving the screed over to the other side.
All garage slabs should be pitched to the center of the slab and then pitched to the front door, at 1/16 to 1/8" drop per foot. Otherwise, you will get standing water from car run off when it rains that you will have to squeegie out :-(
Were I to do such a wide monolith again, I think I would do two screed boards, 10 ft apart, using a 2X6 for the center screed-off board, cutting a 1/8" per foot V in this board, and simply rod off the concrete rather than the side-to-side screeding motion. Does this make sense. That way the two side sections can be screeded off as normal and the center section, once rodded off, will just require a bit more bull-floating to move the cement up to the surface.
I guess I'm thinking out loud here, but its the only way I can think to do this, using managably short screeding boards and keep a pitch to the slab.