Greasy Fork

Sometimes, it doesn't always work as I noticed that the video is still small but it seems to happen infrequently. It could be a thing on YouTube's end as before I switched to this script, the extensions I used to use stopped working a week or so ago. Beyond that, it's been alright.

document.addEventListener("yt-navigate-finish", function(event) {
  setTimeout(()=> {
    var newPlayer = document.querySelector('button.ytp-size-button')
    if ( newPlayer && !document.querySelector('ytd-watch-flexy').hasAttribute('full-bleed-player') ) {
      newPlayer.click()
    }
  }, 600)
})

Try CY Fung's fix. If that doesn't work, bump the 600 figure up to 750 and see if that works for you.