The reason: A large enough pony would be good for me to just dork around on since all I'm really into is trail riding. It'd also be good to teach my friend's son and my little sisters to ride. (If it's not child friendly in the beginning, I'll ride it until it's peaceful enough to do so.)
What I need from you wonderful horsefolk is the breeds of horses that can accomplish this. To help you out, I am 5'7" and around 120 pounds. I'd like to look at something medium since my youngest sibling is currently six-years-old.
Any and all help is greatly appreciated! :)
[And keep in mind that for now, this is just an idea until I know for sure how things are going to work.]