This looks amazing! the level of detail is astounding!
IMO depends on what your goal is, if you're chill and not in a rush, you can make them as big as you want
well the guide is just that, a guide, imo you dont have to follow it exact, it just provides a good structure for those that need it.
and ur not slow that would have taken me weeks if not months to draw lol
it comes thru practice. trying to speed up kind of doesnt work XD at least for me it didnt