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)
http://ironport.com/pdf/ironport_m-series_datasheet.pdf