, Bonnie,
As someone else has pointed out the planting calendar is very helpful in determining when to start seeds and when it's safe to plant them outside. As to size ... I just want them to be big enough to survive the transplant (at least 3 sets of true leaves ... more is better), but at the same time I don't want them too big. Four to 6 weeks is a good estimate for a lot of plants; for peppers, I usually figure about 8 weeks before I can transplant them. They seem to take longer to germinate on the whole and they don't seem to grow as fast as some other plants, at least for me. I don't grow a lot of flowers from seeds, but I'm guessing they would be close to these generalizations.
As for controlling the number of plants you end up with ... well, if you figure out how to fix that, please share ... there's a bunch of us who could benefit from that
