I was taught that a true paragraph is 4-7 sentences. I think that is best for readability, in terms of the comfort level of the eyes of the reader, if you get my meaning. Short paragraphs make the piece seem shorter than it is and easier to get through, especially online. Excessively long paragraphs can be psychologically daunting for the reader.
But remember, each paragraph is one theme, one component of the overall thesis. It may be better to break up paragraphs sometimes, then again it may not. It depends, I think.