"" how big does an apiary have to be to be self sustaining ""
Minimum is zero.
You can join a local club, begin charging for removals, ask the newbees in the club to assist, give them the bees for helping, keep the money.
That said, the true answer is, you can do with bees as you want. You can do the above, or have thousands of hives, or anywhere in between.
Start with 2 or more. One hive is not sustainable. 2 is, but will multiply, so a third box in needed. After that, it's all up to you.