Summarize this article:

About 5 or 6 inches (12.7 or 15.2 cm) is a pretty good size - big enough to get stuck into the knitting, and quick enough that your blanket will start to look interesting quickly!  Each time you finish a row, attach it to the previous row. Eventually you can stop, when your blanket is as long as you'd like it to be.

Summary:
Choose the size of the patches. Choose the size of the blanket, and work out how many squares wide it will be. Keep going!