I suppose they could go in the ground sooner, before they get too big. But, I haven't had an issue with them not transplanting well, regardless of their size. I do harden them off for about a week, a few hours more every day, first in shade, then a bit more in the sun.
I also take prob excessive pains when I do transplant them, I dig a deep hole, put a tsp of Tomato Tone, plus a bit of Epsom salt, cover that with some dirt. I take off the bottom branches of the plant, and bury it deep, as the stem will root as long as it's in contact with dirt.
Most of the time, they take off and grow into huge plants, some of them struggle to grow, but I think it's the variety most of the time. They look wispy or leggy, but do produce some, not like the bigger plants, but still a little bit.
This year, I didn't plant seeds until the first of April, because I actually don't want the seedlings getting too tall and leggy. And, because of that, they'll go in the ground around Memorial Day. We had a freeze last year on May 16th, and this year we had a frost on the 8th, so they might be going in the ground later, but at least they'll be safe from freezes.