* Checking your owner's manual: This is the most reliable source.
* Physically measuring the existing speakers: Carefully remove one speaker (if possible) to determine the exact size.
* Consulting online car audio forums or websites: Search for specifics about your particular 1992 Lincoln Town Car model (e.g., "1992 Lincoln Town Car speaker size").
Keep in mind that even if the original speakers were 5.25", aftermarket speakers of slightly different sizes (like 6x8" in some cases) might fit with some modification to the door panel.