Dimensions for the large cover image are 851 x 315 pixels. The minimum size you can upload is 399 pixels. Be aware that if you upload an image smaller than the "official size" it will be stretched to fit the larger dimension, which may or may not look as nice as you intended. You can drag the photo into position and may have to play a little to find out what works the best.
There are rules, however. Facebook states: "This space is not meant for banner ads or other promotions. Please don't use content that is commercial, promotional..." Do not put contact info, calls to action, or arrows pointing to the Like or Share button, etc. Cover Photo Guideline can be found here.
Ok, obviously FB would like you to use their paid solutions like Facebook ads for things like that. But think about it - a big screaming promotional banner for a cover photo can be tacky and turn off customers.
Here's something else to think about. Unlike the smaller profile picture (which Facebook does not like you to change very often), the cover photo can be changed as often as you wish. FB actually encourages it.
Each time you change your cover photo, it is announced on your wall and shows up in the news feed of your fans. If you tie it into something about your business you want to promote, you can share the photo and add a call to action in the text area of a FB post (not on the cover photo itself).
This way you can be creative and still promote your business within the FB guidelines.
You can check out my website for past hints, articles and newsletters!