There is no optimal length for a paragraph, not in sentences or in words, nor is there an optimal amount of time in which to write it. How long a paragraph should be, and how long it should ultimately take to write it, depends entirely on the claim it supports. My maximum and minimum recommendations are just rules of thumb, and the 27-minute "exactum" is a process guideline, not a product standard. That is, if you're not satisfied with the results of your 27-minute writing session then you are free to rewrite the paragraph as many times as you like. When writing, you should always be looking for the simplest possible statement of what you are trying say, where the "limits of the possible" are defined by the ideas you are expressing. If it is not possible to use at least six sentences, or two-hundred words, or to make a meaningful attempt in 27 minutes, then there is something wrong with the idea you've selected. It may be too big or too small or too difficult or too easy. Again, there is a minimum: I don't recommend publishing scholarship that consists of ideas that are easily expressed in paragraphs that take less than 27 minutes in total to express well in prose. But there's no optimum here. It takes all kinds of ideas to make a paper.