I like to let them get between 7" and 8" bofore final transplant outside, only because that size works best with my method of transporting them. I like to start putting them outside for a few hrs a day when they are just like a few days old if the weather permits. I start w/ just one hr and work up to like 6-8 hrs within about 4-5 days. I know others who let them get to about 14-16" tall before final transplanting. They probably have a higher ratio of getting through the first couple weeks with less tending and revisiting. It's not an option for me to do it that way. I usually have like 90 percent take anyway, and once they make the first 10 days, then its more like 98% make it.