If you roll your mouse over an in-page diagram and you’ll see a button that says Fullscreen.

Clicking on the Fullscreen arrows will open the thumbnail in a full-size preview where you can do various actions

1. Edit diagram (will launch you into Gliffy Editor)
2. Fit to Page
3. Show/Hide Pop-up Notes
4. Zoom bar
5. Close thumbnail (will take you back to Confluence)