Does the clump keep getting ever larger and larger making oodles of tubers?
The clumps get larger as the tubers get bigger and there are more tubers. When you finally divide old clumps you may be dividing 2 or 3 year old tubers, Some very small tubers may be one year old. After about 4 years the clumps do not seem to get much bigger and the center of the clump may rot a bit.
And does it then send up zillions of sprouts as the clump gets larger and larger? Since there are about 20 or so tubers with eyes, there can be 20 shoots, but more often only about half of them grow much. There are limited nutrients for so many sprouts.
Or is there a point at which it becomes self limiting? The self limiting usually is the grower who wants to move it or re-new the bed. They do not spread like bamboo and take over the garden space. A clump two feet wide would be enormous.

Thanks Ted, that's very helpful. I am pleased that most of the clumps I overwintered are strong, healthy and early. I will probably leave them undivided and try overwintering them again this coming winter. It will be interesting to see what sort of stems they make this summer. I'm going to leave most all of the stems that come up but I am topping every stem once above the third node. My goal is long cut flower stems not huge blossoms so I'm not so concerned about channeling all the plant's energy into a few buds. My long term goal would be to have perennial beds of dahlias for cutting that don't have to be dug year after year. i don't know if that's a realistic dream or not.

Swan Island Dahlias has a cutting field of tubers that were were not dug the previous year. I do not believe they let them go beyond the second year. This field does bloom quite early in a normal year but in many years all the tubers freeze in the ground. and they are lost.

I have left them in the ground here for 2 years. After that they are too large for me to lift out of the ground, and extremely hard to divide.However, the stems can get as large as my arm, and don't need staking.

That's interesting flowergirl70ks, do you have a method of pruning them that produces nice stems for cutting? If stems are too fat they fill up a vase to quickly.

I just start pinching when they get about a foot high. I don't fertilize if I plan to leave in the ground.