Normally you can watch youtube either in fullscreen or as a small- or medium sized viewport in a window.

With this style you can resize the video to near fullscreen size while still running it in a window
which is very convenient if you use a multi-display setup and want to view a fully sized video on
one monitor while working with the other.

This script changes video size to whatever the current browser width is.

Mind you, flash videos seems to demand more cpu power when enlarged & windowed than in fullscreen or just windowed without being enlarged.