BB, For what it's worth, I found some wild plants growing on my land and replanted them. They ranged in size from about 6" to 12". I am a terrible gardener and probably did everything wrong. I just dug them up and plopped them into new holes but they seem to be doing great.

Ive never attempted columbine seedlings but with others they usually tell you when they are ready such as growing crooked or roots coming out bottom of pot.

They seem top be super hardy plants for me, so I don't think it is the height you should worry about so much is what are your temps at night and have these plants been grown inside or not. Are they being grown in peat pots? If they are, just put the whole pot in well drained soil and make sure you don't leave them out side if it gets below freezing( unless they have been grown outside).

You might get better answers if you ask on the perennial forum. When you do you should tell them your zone and a little more of where these plants are grown and what they are grown in ie their planting medium.

Good luck