You have little to no influence on the length of a LOR. Programs will not penalize you if your letter writer rambles. I've seen LORs as long as 4 pages (a 4 page LOR isn't any more successful than a 1-2 page LOR, but it's not a deal breaker). However, most programs these days require only a single page. If they want a second page, they'll ask you for it. Some people can write great 2-page letters while others can't do justice to a story that size. Just like with articles, there are some people who are better at writing shorter pieces and others who prefer writing longer ones. However, most people will be better off writing fewer but higher quality words instead of many longer ones. There is no right or wrong length for a LOR. It all depends on what you want to say and how you feel about your subject. Just make sure that whatever you write will be meaningful and well-thought-out.