I don't believe that there is any way to increase the size of the message tracking database. The default is going to be the largest possible based on the amount of drive space in the appliance. Here's a couple of things that may help.
1) Turn off rejected connection handling to save space for actual messages
From the menu: Security Services -> Message Tracking -> Edit Settings
2) Upgrade your appliance ($$)
The C170 has 500GB of total drive space. Larger appliances have up to 1.8TB
3) Add an M-Series appliance. Not cheap either but would likely give you the most space (up to 3.6TB)
in addition to the suggestions Bob made already, if you just have to keep logging data for legal reasons, the recommented solution would be to push the mail logs to another server (syslog, ftp push), and keep them there for as long as you want. They contain all the information that message tracking provides, the acess is different (grep, findevent), but still you'll be able to look up messages or connections later.