Click on the Preferences link and set the display size for the text used in any of the Library content.

Here you can also set the size of the display window for text content: small, medium and large.

Separately, set the size of the text to be displayed in that window. There's a preview that updates immediately so you can check for readability on your monitor.

Likewise you have the choice of small, medium and large for movie window display. The movie is scaled to fit the display size. Again the choice is for small, medium and large.

Once you've set your preferences, click any other tab to leave the preferences area.

See a movie about this (QuickTime 6 required)