I always try to make most chapters the same length (though now and then a shorter or longer chapter seems to creep in). I've been told by editors that readers prefer it that way.

I think that the length of a chapter should be driven by the complexity of its subject matter, and the writer's judgement of how much the complexity should be explored. If you plan a number of chapters that are parallel -- for example, one on a maple, the next on a poplar, etc. -- then the first chapter could roughly serve as a template for the others, and the chapters will be roughly the same length. But if the chapters are not parallel they can vary widely in length.
I did a book on 54 tree species, most of which were allotted 4 pages by the publisher and designer. Each page had to be fully utilized with no "widows" or "orphans". I even had to make captions to an allotted word count in order to not waste any space on the page. It was hell writing some of it, but it looked great when published.

One of the problems with consistent size chapters, space alloted to a species...technique...etc. is that it often makes a book, especially a 'guide', less useful. Depending on the subject, some topics are more useful than others. I might want to know much more about the care and issues associated with a spruce tree than a larch as will hundreds of other readers flipping through a book as a reference. But what often happens is there isn't enough enough 'weight' given to the topics that most people find useful. If you go to a library and look at gardening books, you'll find many sections much more well-thumbed than others.

I wouldn't worry about this too much as you do your first draft. You'll go back and rewrite and reorganize anyway. Sometimes these things are made clear when most of the project is done.

Your editor will also have a say (a huge say!) about the organization. So just get it all down, then go back and start your revisions and rewriting.

A friend of mine who is a song writer said once that most creative people don't have a fully formed idea of how their creation will turn out... they just have the courage or drive to start down that path. The first draft is only the first step down a LONG garden path.
C.L.