It depends on what you are looking to achieve, as far as plant arrangement. Do you want to pack a bunch of smaller plants in an area, or fewer larger ones? For example,....in a 4' x 4' area, you could pack in 5 rows of 5, smaller plants, or 4 big ones(30"+), and still get about the same yield, with either setup. Honestly, I think it's a better idea to go with fewer plants, being that it's your first grow, because it's alot easier on a new guy. Jumping right into growing, with a shitload of plants, is alot of work,(feeding, flushing, transplanting, etc..) and risky.
Also, a week old plant should be at least a couple inches tall, but as mentioned, can vary alot. Basically, if you have a toothpick-looking plant with 2 tiny leaves, at the end of week 1, it's not growing well. Aside from that, there's too many variables to say for sure. I'd say 3-4" is average though, for typical strains, if I had to choose an answer.
Speed them up? Yes. Don't skimp on lighting, and make sure you have a good growing medium. Light, is everything. Light, is ultimatelly, what powers the plants' growth, and all the care in the world won't help, if your plants don't have the light intensity needed to grow vigorously. Shoot for 5,000+ lumens per square foot,(LSF) if you want excellent performance. Keep 3,000 LSF, as your absolute minumum, and go from there. Your growing medium should have excellent drainage, and be well-aireated. Also, make sure the pots are big enough to contain their root system without becoming rootbound. I use 3 gallon pots, with 30-36" plants, and have never had any problems with rootbind. I used to use 2 gallon pots, and never had a problem, either. I switched, because the bigger pots hold more moisture and nutes, making them easier to maintain(exrending watering/feeding times). Light, is still the most important factor, so worry about that first, if you're looking for speedy growth.
