They need to have formed at least two true leaves and soil temp should be 65 degrees. Here's a good article: Chili Pepper Growing Tips

Bela....planting time is determined by your local conditions, not the size nor age of the seedling.

If you live in the US, you can talk to your local university Extension specialists to find out when the recommended planting dates are.

I'll start my peppers under lights in a very cool spot....my garage. They'll grow slowly but will be nice and stocky (sturdy). My planting time for peppers and tomatoes is in late April/early June and the peppers will have been growing for about two and a half months.

Everybody has their own strategy, depending upon their set-up inside, space, and their local climate. What is your general location?

Agree with rhizo - it isn't size that is the determining factor. Anything from 2 true leaves on can be transplanted. I prefer 6" tall transplants but even they will have to wait until the conditions (soil temps, air temps, etc.) are right. That is true for most any plant.

My soil temps don't usually allow for pepper transplanting until late May or early June, about a month after my tomato plants go in since they don't need as warm soil. In your zone it would likely be earlier but learning how to measure your soil temp is the key.

Dave

FAQs: when-and-how-should-i-transplant-pepper-plants

I know for myself, I will try to wait until temps are not expected to go below 50 overnight -- if I wait longer than this, it is likely I will be cutting the season too short and missing out on the peak sun season. I grow varieties that are ok with this. So I start my seeds towards the end of March. You don't have this issue, and can probably start now if you haven't, though I don't know your frost dates and such. Pepper plants are very slow growing.

It is also important to harden off your seedlings properly, and not to plant them out until they are fully hardened off, even (or especially!) into ideal conditions.

I am also in Zone 8A. I grow all my peppers in pots outside, so my soil for them warms more quickly. But even then, I don't usually transplant them out until late April. You should be able to lay your bare forearm on the soil for a few minutes and your skin should not feel cold. Peppers are slow growers and they need very warm soil to germinate, though cooler for growing on. I start mine 8 to 10 weeks before the date I expect to transplant them outside. It won't be long!