Not so much a certain size, but while they're small, as long as she has an awesome filter, they can stay in the tank for now. They do need more room, though. A 10 with 3 koi and a 55 with 8 koi isn't enough room for any of them, as they grow pretty quick.

If a large fish stays small due to it's living arrangements, you're actually stunting it's growth. Of course nobody wants to do that...

Hope I explained welll enough.