I would not send anything longer than one page as that is a known good amount of info that a person can read without pausing. You want to make life easy for the officer, not make him read a novel about you. Good English is simple and straight to the point.
Personally, I think it would be difficult to provide a good feel for the relationship in a single page. And it depends on what other proofs you are providing as well. My husband and I had been together for many years, married for over two years, when he filed for residency, but we had little to show in the way of proof because we're not social people. We just hang out alone at home when we're together. When he was gone, he was driving a truck over the road and we would go for weeks with no contact. That was the reality of our life. Our letter was thoughtfully put together and, in my opinion, was extremely important in providing insight into our relationship. A single page would not have been sufficient.
If there is lots of other evidence, I know some people have loads of chat conversations and photos and stuff, a letter might not be necessary, but it can be very useful. A few pages certainly isn't excessive and it could make all the difference.