The standard cliché regarding sentence lengths in a novel is to vary them, to keep the reader interested in the action. I agree, but with this caveat–the sentence lengths and variation should also suit the pace of the plot. For example, in my latest novel, sentences are longer when the pace is slower, and shorter when the action or drama increases. The following illustrates this contrast (using ProWritingAid). You can guess which chapter contains fast—paced action and which is slower? This will why I ignored the red dot warning in in chapter 15! Such analysis is a useful guide to writers.