In general, the cover letter is not limited to the length required, but you think about it, if you are HR, others a few pages to get something for you, so long as you see, you can have that kind of patience to see Finish?
So I think, should be controlled in a cover letter, a text is also not too much, and not too little. right on the line! really do not understand, then to see what other people how to write. And, last, A4 Paper printing can be!