Here's how you can get a free script for Firefox, Chrome or Opera that will let you set a default video size and quality for YouTube videos.
1. Go to the Yet Another YouTube Script page.
2. Click the Install button.
Here's a description of Yet Another YouTube Script:
"A lightweight and non-intrusive userscript that control autoplaying and set the preferred player size and playback quality on YouTube."
For more, see the original article at the link below.