To hold your readers’ attention, you want variety in the length of your sentences: some short, some medium, and even the occasional long one. But the more complicated your content is, the shorter your sentences should be. Instead of one long sentence explaining a difficult point, break it down into two or even more sentences.
Big blocks of gray print put people off. Look at a newspaper (quick, while they still exist) and notice how short each paragraph is. If several of those were combined, readers would likely give up before starting to read the articles, merely because of the way the print looks on the page. White space invites your readers in; those big gray chunks of print turn them off.