Size of outdoor box turtle habitat?

I have two box turtles and they are in a 50 gallon tank which I know is too small. So I'm making a list of what I need for an outdoor enclosure and I'm not sure how big to make it. Please don't say as big as possible. I need an idea of how much space is enough but not too much for two box turtles. Also what is the best to use for substrate? I'm giving them a little pool but I don't know what to use as substrate. Also, my brother is giving me his russian tortoise and I was going to make a separate enclosure for him. So same questions, what size for two russian tortoises because I may buy another one. And what substrate is best for him?I read that a mixture of play sand, top soil, and some reptile moss is fine. Is that fine or is there something better. Any other advice on an outdoor enclosure for box turtles or russian tortoises would be appreciated.