If you are logging all connections, then no Firepower appliance is going to have the capacity for a year's worth of events on anything but the smallest network. The high end appliance can store 1 billion events but a medium size network an generate that in as little as a month.
Here's a reference to the appliance limits and how to configure the database allocation for event types:
https://www.cisco.com/c/en/us/td/docs/security/firepower/623/configuration/guide/fpmc-config-guide-v623/system_configuration.html#concept_C94E9492C76E4CCC9100B3139C7CF771
A given connection event consumes about 700 bytes per event on average, but that is highly variable. If you do the math and map that against the appliance types, the resultant database size is (roughly) between 33 and 652 GB on the FMC (FMCv vs. high end FMC 4500 appliance). cisco does not allow you to change the overall database sizes - only allocate among the various event types (connection, intrusion, file, security intelligence etc.).
Unless there's a specific legal or regulatory requirement, few organizations really need to store more than a couple days worth of connection events. If you really need to do that, then I suggest looking into a third party tool like Splunk (consume vents via eStreamer) or one of the open source syslog management solutions like Graylog or ELK Logstash (consume events as syslog messages).
