For frequent use, browser extensions like Image Downloader or profile picture viewers for Chrome can add a "View Full Size" button directly to your Facebook interface.
On some browsers or configurations, this option may be missing. You can try right-clicking and selecting "Copy image address" instead, then paste that URL into a new tab. On mobile devices, this method typically works best on desktop, so you may need to switch to a computer.
View Facebook Profile Picture Full Size: 4 Easy Ways (Desktop & Mobile)