The standard Facebook Timeline cover resolution is 851 x 315. You can of course make it bigger. If it's bigger than 315 Facebook automatically center aligns it vertically and you are able to move it up and down to fit your needs.
Me though, I just experimented a lot until I was satisfied. This picture has a resolution of 1217 x 450 and worked good enough. I also made another Timeline cover with even a bigger resolution of 3680 x 1360. Which is really absurd but it gets the job done and the results are amazing. You should know though, these kind of resolutions for images take a lot of time to upload.
I hope this was helpful enough for you and sorry if you're still confused about it.