There is no "perfect" length Guide. Some of our best Guides on the site have been some of the shortest (and some of the longest). One of the top performing Guides currently is one that has only 7 sections, each section composed of a single video. Other Guides are much longer, with upwards of 25 or 30 sections. It all depends on what the topic is and how the content is best organized.
Generally speaking, Guides work best for covering topics that require more than just a "Top 10" type explanation--Buzzfeed, Linkedin and Medium are probably the better places to publish listical type Guides. Youtube and other sites like Wikihow, Quora, or Ask.com are probably best for short-form answers to very specific questions like, "How to tie a tie".
Guides.co is much better suited for How-To's that "go a little deeper", and that evolve over time based on user feedback or changing conditions.
More important than length is how the content is structured. Guides are rarely, if ever, read chronologically start to finish. It's much more likely that a Guide will be read in small chunks, from a mobile device, in the moment that your Guide is relevant to them. They will probably jump around the pages in your Guide in a popcorn like fashion, and only read the pieces that interest them at that moment, and it's important to keep that in mind as you write your Guide so that the information in the Guide doesn't get lost in translation.