Planting out small seedlings

[Csross]

I'm starting tomato seeds for the first time, and only have experience with planting out the stocky, 8-12" tall plants from Home Depot, etc. I don't have the time/space/permission from my wife to buy a shop light & heating mat setup this year, so I'm hoping to start a dozen plants in my kitchen window. I've got a bag of Gurney's seed starting mix, which is coir-based and contains some fertilizer. Here in MD, we have a last frost date of May 5, so I'm thinking about starting seeds around April 1. That gives me 5 weeks until plant out, which I'm hoping is long enough to get them going, but not so long that they get spindly without artificial light. How tall can I reasonably expect them to be, and does that time-frame sound right? Most of what I read suggests starting 6-8 weeks before plant-out -- will they be too small at 5 weeks? Is there a downside of planting small seedlings, other than that it'll take longer to get fruit?
Thanks very much!