How tall should my morning glory's be being i plant them in the ground? or does it not matter as long its warm

I'm in zone 7b if that helps any.

How tall should my morning glory's be being i plant them in the ground? or does it not matter as long its warm
I have always put the seed directly in the ground and they do great that way, but the ground should really have warmed up first. Just make sure it's well past frost time or you'll need to cover the seedlings.
Reply:after last frost date . height is irrelevant