Final answer:
Route 53's Geo DNS feature allows a social media company to route traffic to server resources based on user location, customizing the coverage area dynamically.
Step-by-step explanation:
The feature of Route 53 that allows a social media company to dynamically alter the size of a geographic area from which traffic is routed to a specific server resource is Geo DNS. Geo DNS enables traffic routing based on the geographic location of the users, so the company can tailor the reach and performance of their server resources to serve users more effectively within specific regions. The other options such as latency-based routing, weighted routing, and health checks provide different functionalities.