The main reason I’m currently looking at going smaller than A5 for my main notebooks are simple. Most of the time I don’t use a page of A5 as one page. Or not all of it.
One option would be to do to A5 slim. Another is something like A6. Or anything close to them.
For all of my writing, lists etc an A6, Travelers Passport, pocket size etc (just talking about all of the almost the same sizes as one here) either a single or a double size of them would work fine. Most of them are written either my dividing an A5 in a 2x2 grid or in 2-3 lanes anyways. Or for writing I just waste a lot of room.
My planners are a bit more complex to get that to work with something that small to get the layout I want: weekly, horizontal, same size all days on one line. For this I think it would have to be a double spread layout if I wanted something small.
Part of my motivation for this on the short term is to reduce the size and weight of the notebooks I carry. Plus that if I get a bit smaller than A5 it is so much easier to fit notebooks in packing cubes, slings etc. The options are so much larger even if I just scale it down to an A5 slim.
The solution in a notebook world would be to use a Hobonichi Weeks or full size Travelers Notebook refill. When I get a binder it leads it me to consider how I want to do things. Multiple binders is one option, another is to not have everything in the binder, or to go for the option that is the best for all my constraints.
But smaller notebooks going forward will be what I’m going to play with next year or as I use up what I have.