With me it starts with which way I sow them in the first place!
To explain; if I sow them in trays or pots I prick them out as soon as I see real leaves!
If I sow them in cells I can prick them out Just after germination because I transplant the plug without disturbing the root system.
I find with the latter method(plugs) the plants are not checked and they do not spend much time in the fertiliser free seed compost and get the benefit from the fertiliser in the potting compost immediately.
Once they are in to 3" pots I just grow them on until planting out time.
Regarding size at planting out time is dependant upon variety and the weather conditions from pricking out time to planting out time, i.e. how well they have grown.
So some years they are 3"-4" tall and other years 6"+
So that's it basically, meaning I like to get them out of the seed compost ASAP and get them feeding, and I don't plant out until things are getting warmed up, they do not like the cold!
So sometimes it is better to plant out quite late on in the season,rather than when conditions are cool.
By doing this I find they do not sulk after planting out, as they would if they are cold.
So I play the weather.
I hope that answers your question.....Tg
