Starting early indoors?

Really new to seed starting, and I have been reading the instructions on a lot of the seeds I have bought. A lot of them say to start early indoors 6-8 weeks before the last frost.
My questions is, if I do that, how big can I expect the plants to be after the 8 weeks?
I know that is too generic a question, so let's say I am starting Petunias and Zinnias. I shouldn't expect to get nice bushy plants like you buy in the 6 packs right? If not, can I start them earlier, assuming I have space for more lights etc?

Here is the new link for the winter sown section that is being done, hope this helps:
http://davesgarden.com/community/forums/t/794314/

I started 48 Purple Prince zinnias in a Burpee seed starting tray about about 8 weeks before planting them out. They were an afterthought and I didn't pay much attention to them. They were only 7 or 8 inches tall in those small cells. The potting mix in the Burpee self watering trays (which I don't really recomend as they are very thin plastic and can't be moved without difficulty) has some gelatinous binder in it which, I think, helped them survive being so root bound. Even had them outside under a tree for a week or so. Anyways, once in they perked right up and took off. Below is what they looked like in mid August (I put them in late May). I wouldn't worry must at all about zinnias. Very durable plants.



This message was edited Nov 30, 2007 11:39 PM

The producers of the sixpacks you buy use a lot of tricks with fertilizer and temp changes to get the plants to the selling point. The main things I have found that produce a good transplant from starting indoors are plenty of light and some decent fertilizer. I use shoplights. Keep them 1-2" above the plants, and they are happy. I also use foliar feeding with ferts like Omega 6-6-6, an organic fert. It is also a good idea to brush your hand over the top of the seedlings every day--helps strengthen the stalks. Some people use a little fan for that instead.

Thanks for all the info guys.
My last frost date is March 3rd, so I know according to the seed instructions I can start them 8 weeks in advance. That puts me in January. If I wanted to get larger transplants can I start them now? Is there an important reason to wait?

Well, lots of people, including me, believe that you should not start a seed for planting out in spring until after the winter solstice, because then days are lengthening and the seeds seem to know this. Call me superstitious, but I usually wait until that time is well past, even though I am always raring to go way too early. If you start things too early, they will be stunted because they will be stressed when they are juveniles. They will typically be smaller than things you start later. I don't know why this is, but I suspect once again it has to do with the day length.