UPDATE: I just learned that the iframe width is 480 px only when viewed over a secure connection, possibly because I am using social-server.com rather than having my own ssl.
I’ve designed a page tab to be displayed in a Facebook iframe. I designed it with 520 px in mind as the maximum width. However, Facbook loads the page in an iframe that is only 480 pixels wide.
It looks like it is supposed to be 520 pixels wide, because there is extra white space between the iframe and the next column. Also, I checked other pages with other custom tabs, and their iframe is 520 pixels wide.
So I wonder if this is a new limit on new custom tabs. I really appreciate your help!
Custom tab width is 520px, perhaps you need to set body margins to 0px in your css. Check this out.