15 % for 100 efficiency? So a City probably.
As far as my understanding goes the population and the size of the estate are only indirectly linked. They are linked in that as the estate size is increased, the population within the estate increases while efficiency drops. But, a change in population does not cause efficiency to increase (or decrease) to my knowledge. I've certainly not seen that happen in game. If it did though, an increase in population should decrease your efficiency.
From my two characters on testing islands:
For the city of Vur Hagin, an estate of 40% is 4,644 people and efficiency is 73%
For the rural region of Dantooine, an estate of 50% is 6,046 people and efficiency is 71%. If population was max in Dantooine the estate would have 7,750 people in it.
As for helpful hints: If you have empty estates you are only going to collect half the gold you would in an estate with 100% efficiency. So, close some of those down and give you or some of your knights bigger estates. 75% efficiency in an occupied estate is a lot better than the 50% you'll collect from wild lands/empty estates. You'll want to still keep some empty estates to attract more knights but there is no reason to have 4-5 empty estates with 100% efficiency.