Cheap school house to build

This is a very niche question but I’m starting a daycare and I really don’t want to run it out of my house if I don’t have to…. I have 3 acres of land so I want to put a small school house on the property to have a daycare in

How many square feet do you think the building would have to be??

Also does anyone here have one and could tell me how much it cost for you to buy/build it

I know that’s so niche