YouTube has several useful icons that resize your player, but none make the screen smaller. Smaller screens come in handy when you'd like to view a small player while you work on other tasks. Your browser has special keys that can make your YouTube video larger or smaller instantly.
Make Your YouTube Screen Smaller
When you press "Ctrl-minus sign," your browser shrinks everything on a Web page by a small increment. Press this key combination repeatedly on a YouTube page until the video is as small as you like it. You can then resize your browser if you like so that it is barely large enough to display the video player. Drag the browser to one of your screen's corners, and you can work on other tasks while your small video plays.
Make Your YouTube Video Larger
To make the player fill your screen, click the full screen icon at the bottom of the player. Exit full screen mode by pressing "ESC." You can also click your YouTube player's theater mode icon to make it larger without filling your screen. This viewing mode is handy when you'd like to watch a larger video without losing the ability to see other parts of the screen. Return to the player's default view by clicking "Default View."
YouTube Video Quality Settings
YouTube adjusts a video's quality automatically based on your Internet connection speed and video player size. For instance, if you have a faster connection, you might see higher definition videos than someone with a slower connection. Although YouTube's default quality mode is "Auto," you can change that by clicking the player's gear icon and selecting another quality setting. If you make a video bigger and experience slow or erratic playback, click the player's gear icon and select "Auto" or a lower quality setting.
YouTube Resizing Tips
Larger YouTube videos can help you see small details in a video and make it possible for more people to watch it at the same time. Although images on-screen get bigger in full screen and Theater mode, they don't look as sharp as when the player is smaller. Conversely, you lose the ability to read on-screen text and identify objects when you make a YouTube screen too small. If you have trouble watching a YouTube video in full screen, visit the Adobe Flash Player Help page and follow the instructions to check your player's version. If you have an older Flash Player, follow the instructions to install the newest version.