I am sorry if this is in the wrong area...

but I looked through alot of forums and have had little to no luck on finding the 'recommended' size of a custom background to put in a pets profile.

Goodness I feel like such a moron lol, and I'm normally the computer techie in my family for this kind of thing.

Well, it totally depends on your personal preference. I aim to make layouts that are 1000 pixels wide or less to accommodate most screen resolutions, and then have some kind of repeating background to fill the rest of the page for larger resolutions. The height doesn't really matter.