If you have never divided before, spring is best as it is the easiest time to see the eyes when the new sprouts form. If eyes are difficult to see, we suggest dividing the clump in half, or quarters. Not all tubers will have an eye. Cut surfaces should be allowed to dry thoroughly overnight before storing, or planting if spring dividing. Tuber size does not affect plant growth so even the smallest tuber will produce a full size plant if it has a live eye.
Different varieties produce different size and shape tubers.
See figure 2. This site and its contents are 2001 Swan Island Dahlias.