I'm glad you're thinking about expanding your art, because no offense to anyone here, but sigs are completely useless. Not to be mean or anything, but they're only purpose is to be added to the bottom of your post on a forum, and that's it. In any real world situation, they're not worth anything.

As far as going from sigs to large pieces, I think the biggest mistake you can make is trying to make a large piece using signature techniques. Sigs are all the same really. A render, blending, brushes, finished. But with a large piece, it's all about creativity. Plus you have so much room to work with that you cant just slap a render in the middle and use some brushes for the entire background like a sig.

If you're going to get into large pieces, I would select a style that you really like. For example: photo manipulation, abstract, pixel art, vector, etc. Choose one and work on developing that style into your own. It's fun to make wallpapers, because then if you really like your outcome you can throw it all your desktop and see it all the time (as well as others who use your computer).

But yeah, definitely get into the larger pieces. Maybe even web design. Something you can make money off of. Not saying you should chase money or anything, but why develop a skill (making cool sigs) if it has no potential payoff later down the road?