If you go to a granite slab dealer you'll find that the slabs are generally about 5' x 9-10'.

Counter size

longest slab you can buy are 10'.

The ease of getting it in the house + shape + weight are limiting factors. Weight was major in deciding if mine would have a seam. I have a peninsula/bar area that would attach to a length of counter, they decided to put a seam.

Your fabricator has to come to your house and follow the path the counter will take to your kitchen. Remember that it will take 2-3 big guys to carry your slab from the truck into your entry way, up any flights of stairs, turning all corners, avoiding ceiling light fixtures, through doorways, and into final position. If your ceilings are standard height, that could also be a limiting factor. But it's your fabricator who has to have the final say.

It also depends on the layout. We avoided seams in our L-shaped counter: one piece for the long run on the sink side and extending a bit around the corner.
Next comes the range top. Then there is a single piece of granite to finish off that side. The fabricator was nice enough to let me guide him about cutting the slab to make this work.