When you visit YouTube and hit play on a video, the video player offers Small, Medium, and Large size options. You can select either the small or large player, or go full-screen. With the Youtube Resize extension for Chrome or Firefox, you can drag to resize the video player to any size you'd like. It's helpful if you have a particular video player size preference for your particular display that falls outside of YouTube's three options.
You'll notice that with the extension enabled, when you mouse over YouTube's video player, the cursor turns into a sideways arrow. It lets you drag and drop to resize the player. The extension places your selected video -- no matter the size you choose -- at the top of the page, with the related videos column pushed below the video, falling in line with the title. It's the same layout you get when you choose YouTube's large video player.
Also, the Resize extension does not work with embedded videos, only with those accessed on YouTube.com proper. And it works only with YouTube's flash player and not the HTML5 player you can enable.