It should not sound over enthusiastic, though. Professional tone needs to be maintained. Generally, it is not a norm to write anything negative about a person on such letters. However, you can mention a few critical things, if required, so that the letter looks authentic. Recommendation letter, like other formal letters, should include date, place and time. It should have a small introduction part, stating purposes of writing the recommendation letter. Generally, professional reference letters come in 1-page length. But, it could be maximum 2 pages in terms of length.