Having just wrote that, I think I answered my own question. The problem is, if you've just come here via Facebook, or a search or Reddit, this is all you'll see. I'm not above doing larger commissions, and I have in the past. I think it's just awareness.
To be fair, a large reason I do the "sketches" is a time issue. Were I to take on a larger commission, it would take me longer. I can do a portrait in 30 minutes. Something more would take days, weeks maybe. I do this for fun as well as profit, but I don't (yet) do it for a living.
To that end, if you are interested in something a bit more formal, a bit larger, a bit more artistic, I'm completely willing to take a shot at satisfying your request. I've also added a link on the sidebar that takes to a collection of all the artwork I've done to date. I try to keep it as current as possible so check back often.