There is an addition on my house that is a bedroom with a garage underneath. The wireing is jacked up and I was thinking of installing a sub-panel in garage to service the bedroom and the garage. Is this advisable? If so, how many circuits should I put in the panel? There is one slot open on the main breaker and I have a 100 amp service. Also, what gauge should I use to connect the sub panel to the main panel?

This is entirely subjective, but IMO it's a good idea.

A typical 50a sub-panel would be fine.

You will be using a 100A rated panel. They can be had up to 24 circuit (sometimes even 32), but I would advise staying around 12-16 depending on what you find.