When I was in highschool and crushing on a girl, I ended up working at her father's goat
dairy farm.
They had a donkey named Abner and I have been enamored by donkeys to this day.
I believe the rule of thumb with donkeys is half an acre
EACH. I believe this is considering pasture grazing requirements but don't quote me on that. I do not know what it is for Minis but I can assume that you might be right on that line. I tend to err on the side of caution and would encourage you to see if you can squeeze in another quarter acre fenced.