Best time to transplant?

Ok, I started with seeds this year and my babies are doing pretty good. I didn't have much soil left from last year so I filled my pots only half. I was wondering, when is the best time to transplant?

I don'T want them to get rootbound and I don't want to them be too stressed by the operation.
Should I do it before/after watering? In the morning/evening?
Can they be too small to transplant? They're about 8 inches tall in 4 inches of soil. They are not stretching and looking perfectly healthy.

Thanks in advance! Pics coming, I promise

i trans plant like 1 hour befor the lights go off

I'm outdoor

One hour before lights out, or one hour before it's completely dark.

The dark period allows the plant to relax and "get over" the stress a bit better as it has less to focus on. (it won't be photosynthesising.)

Once you've transplanted water her thoroughly.

A plant is never too small to transplant. (unless it's a brand new clone with no roots)

Ideally you'd want to transplant about a week to two weeks before putting a plant into flowering as you want your plant to have the least amount of stress during flowering as possible. (stress causes your plants to pause growth of buds and concentrate energy on stabilizing itself)