Safari doesn't handle that as well as it should.
Install Firefox instead (you may have to head to System Preferences > Security > General and set Downloads temporaily to "Anywhere" to accept software from the web)
Then install the add-ons NoSquint and Theme Font & Size Changer, you can mess with those to make a perfect sized web pages and menu bar type size to make the perfect web browser for your large screen.
It is especially valuable tool for visually impaired people and wide screen users.
NoSquint allows you to adjust the text-only and full-page (both text and images) zoom levels as well as color settings both globally (for all sites) and per site.
The Firefox Toolbar also can be customized and a Persona added in the background or a theme even.