Peter, there are lots of Coleus available at the nurseries now. Why don't you forget about starting from seed for this season, and buy some transplants. It's getting awfully warm for trying to plant out seedlings. You'll have nice size plants so much sooner if you do. Start seeds in October, grow them through the cooler months, and you'll have a garden full of wonderful = good size transplants next spring.
You'll also be able to take cuttings of the ones you buy, and carry them over for new plants next spring.
My Coleus are all over 2ft tall and growing like gangbusters now. This picture, the pot is 16in. for size reference.
