I started with a king size and now realize how dumb that was. I didn't do a bad job, but it was a big investment in time and money. After doing many more quilts I can see how that first quilt could have been so much better. And it was a gift so I wish it were better.
Start small with a baby or lap quilt. If it's reasonably good it will sell. Or you can snuggle under it while you work on the next one! When you are happy with your work you can move on to bigger. When I got to the craft fairs (we have a lot of them here) it appears that baby items sell well if they are priced right. People are more picky about buying quilts for their own beds.