The maximum size a Black Pudding will reach before allowing itself to 'reproduce' through splitting is eight to nine feet.  As for the minimum, this is not specified. However, a Pudding will not split if its hit points drop below ten. If you use the hit dice at their maximum to be generous, a Pudding can have at most 130 health at the maximum 8 to nine feet. These hitpoints are rounded down and halves when split, so we will use the same for this monster math.  With this criteria, a slime can split 130-60-30-10 // 9-4-2-1. So should a Pudding not take any damage, as they are immune to slashing damage, they will at most be able to split into eight one-foot creatures with 10 hp.