Hi Bonnie, and
My problem is the opposite: not "how big is big enough to plant out", but rather "how long can I get away without potting up or planting out?"
Try to harden off and plant out after 2-3 pair of true leaves, but before the seedling cell is root bound. That seedling will get unhappy pretty fast once root bound. Maybe flowers are more tolerant than vegetable seedlings, I don't know.
When I run out of space indoors under lights, SOMEthing has to go outside. I really need a cold frame or small hoop tunnel.
Maybe this is less of a problem in Zone 9 / Arizona, but I can't plant out until I think Spring has settled down. We might have months of pretty warm weather followed by the only hard snow of the year.
Some years the slugs are so active that a seedling has to be big enough to give up 1/2 of its leaves and still survive.
