Hi Patty,
Big clumps are hard to handle and will produce fewer blooms of lesser quality than one or two healthy tubers. (Says she who left 6 clumps in the ground last winter.)They keep reproducing tubers each year, and get more difficult to handle.

Dividing is a great way to get more tubers for planting or sharing. You should be able to see which ones on the clump are developing eyes or growth buds and make your cuts accordingly. I think people advise if in doubt about dividing individual tubers,just cut the clump into halves or quarters and hopefully you'll have something that grows out of each bunch.
I would let the fresh cuts dry over and even dust the cut area with sulphur before they go into the ground though.

Good Luck in your second year of dahlias.
Poochella

Here is a link that might be useful: Dividing Tubers

This is my 4th year growing them, in the ground and in pots. I did not divide 3 clumps that I put in the ground a few years ago and last year they were pretty lame indeed. The other ones that I divided did just as well as when I first bought them.
So in a nutshell, my completely un-scientific opinion is to divide em!