The length of the letter should not be the issue. If it is a strong letter and shows that he/she truly knows you and it is not just a run of the mill letter, then the length does not matter. The letter I got from the DO i shaddowed was at most 15 or 20 sentences, and was shocked to see how short it was when i first saw it, but after reading it, I knew that it was my strongest LOR.
Long story short, the content is what is gonna matter not the length. Also, i think that there is a difference between a concise letter and a short letter. Concise letters don't waste the reader's time, which they love. They hate fluff. Just my 2 cents though, take it with a grain of salt or however you would like...