To make any website fill the entire screen, which is a little ridiculous on an iMac with a 27" screen, just grab the bottom right corner and make the page as large as you want it.
Alternatively, click the Green button at the top of the page and Safari will automatically adjust the size of the page to that of the website.
Using the Green button method, you do not get all the white space at each side of the page.

Go to Safari Preferences > Advanced and change the minimum font size to something larger

Go to the Adobe site and download the Flash uninstaller. Once you have run it and Flash has been removed, download and install the latest version of Flash from the Adobe sight.