To make reels full screen on story on your iPhone, first check if there's a 'Expand' or 'Full Screen' option when you play the reel. Sometimes, swiping up or down might also do the trick.
The way to make reels full screen on story on an iPhone is to search for the full-screen button, which might be in a corner or shown when you tap on the screen. Also, make sure your phone's settings for display and media are configured correctly.
You can usually do this by double-tapping the reel or looking for a full-screen icon within the interface.