I don't know what the average weight for Calls is here in the states, but size is one of the easiest traits to breed for in ducks. You would probably be better served to buy the best quality you can find there and selectively breed from that (rather than import eggs or birds). I have been doing some breeding tests for size and altering size using Mallards and I can more than double the size of a Mallard in only two generations. Going down in size is a little more difficult, but not much. All of the bantam ducks I have worked with are influenced by feed intake as much as genetics, if not more (with respect to size). I would get the best stock you can, select for size (if that is your top concern over type), feed a good but restricted diet, and select for size again in the next generation. You will be very surprised at how quickly you can increase or decrease.