<p>Well, I think half a page is OK, and one full page is fine.
And I told so to my professor.
Am I right?</p>
<p>Sorry if someone already discussed this before.</p>
<p>This is purely a guess based on the recommendations I have seen.</p>
<p>I would think it would be difficult to write a good recommendation in a half page. You want the recommender to cite some specifics to back up their recommendation, which takes space. Plus, figure the double spacing.</p>
<p>But your recommender should know this already. I would be a bit concerned if my recommender asked me how long it should be! Maybe he/she is just eccentric?</p>
<p>Oh, I haven't thought of double spacing, because all recommendations are sent electrically mostly as plain text.</p>
<p>It seemed that my professor wanted to make sure he would keep an appropriate length. Maybe he is not very familiar with the online submission stuff... I'm not sure.</p>