I am confused about the use of Space vs Max-Width. I’ve mostly been using Space for grouping a set of related stacks, which I name so they can be hidden during editing. I used Max-Width when I want the stack to be less than the width I have set in Spaces.
Am I on the right track?
You’re probably on the right track. It helps to think about things “responsively”. What happens to content when a page is on a narrow screen? What about on a wide display? Without “Space”, the content will go right up against the edge of the screen or container on a phone. Conversely, without Max-width, your content can spread really, really wide on a very wide monitor.
The two tools do nearly opposite things. Again, they are both handy when you begin to think about how your page reacts when changing size.
(edited typos)
Thanks. I use two of the presets for each Space but I’ve wondered if there were a global setting for this, so I would be sure I’ve set them all the same?
I’v thought name of Max-Width should be just Width?? 
Width would imply that it is the same at all times. Max-width means that the maximum the width cam be is what you choose, but will become fluid, and responsive below that width.
As Adam mentioned, Width would really be something different. You’re not setting the width, you setting an item’s maximum width. It can shrink smaller but not grow larger. Width would always be the same size.
As you get more used to thinking in responsive design, this will get easier.