how much room in a yard is mandatory for one around 5 years old?10 by 10,20 by 20 or what?

The larger the better. What your yard can afford. Because I have the 3 I have half of my back yard devoted to them. These are fosters so I can down size or increase as need arises, just by moving cinder bricks.
The red line shows were the tort pen is, all the stuff behind it is now gone. That is a large tent type shed or portable garage I think they call it. I believe it is over 20 feet long and I could easily set two or three in the tort enclosure