You don't need a drake for eggs, or for anything but fertilizing them (and some extra drama from time to time!). The drake should be close in size to the ducks - not too big. If you just want a drake, first, realize he could spend too much time mating with one or more of the ducks, harming them. Not real likely, but it can happen and you need to know to look out for that. Also a drake can be trouble with chickens - he could seriously harm them if he tries to mate with them. When they start laying depends on their age, food, environment, and the season. So many variables. How old are they? Are they getting layer feed yet? What is the temperature in their environment?