Riding Donkeys? What Size?

I've seen some REALLY nice donkeys for sale around where I'm going to be living... and for good prices. I'm just wondering... how big do the donkeys have to be in order to ride them?

I know that the age has to be somewhat older (four and five). But I was wondering if smaller donkeys can't be ridden as well...

I know I'd need a somewhat taller one (10+ hands high)... but I'm just kind of curious... :P

They can comfortably carry a lot of weight for their size, more than a horse. Unfortunately, I'm no expert, so I can't give you exact numbers.

I once rode a donkey that was probably 11 -12 hands (same as our pony). I know someone who breeds donkeys and people bigger than me ride her donkeys. I weigh 52kg and her donkeys probably average 11 or so hands.