I've been removing as much as possible, but I've noticed that the shirts done professionally have left inches around the desing. How much should I be removing when I finish a project?

You must trim it as much as possible, so that your project is clean. So you can make the difference!

I had a friend that received a monogrammed bathrobe & they left the whole sheet of stabalizer on the back. She called me & asked if it could be removed :)

I try not to leave more than about 1/4 inch when I trim. I think the reason you see more on storebought is that they don't take the time to trim closely. I have purchased items where it is obvious that the stabilizer has not been trimmed at all.