Lots of responses here, and some great points. Thanks, everyone!
I am at a point where I care about print size more than page size.
I prefer that smaller books not be so thick that they have no hope of lying open. My Savage Worlds books are like 6.75" by 10.5" and they seem to work fine.
I feel this. The first harcover I ever produced has slightly oversized text (by which I mean an 11 point font instead of 10 point) and a couple of folks commented on it, but it sure is easier to read.
I'm looking at a pair of zine/digest sized books on my shelf that both top 200 pages; a 230 page hardcover and a 300 page softcover. I hadn't really thought about it before, but the hardcover definitely stays open easier, if only because I don't wantt o break the spine on the softcover.
On the other hand, I also have an 80-page digest sized book that is perfect bound, and I basically never use it even though it has a ton of great content, because it won't stay open easily. I think it would work better as a shorter 8.5 x 11 sadde stitch book, but that's not what the creator chose to do.
I think it also matters what you're writing it for. For example, with my current KS, I decided to make the books match what the size of the edition is. OSE and Shadowdark are smaller than 5e books for example. So I wanted that to be a consistent experience for the player.
The key is to ensure the print size is still readable.
I like that thought process: make stuff match the main line you're writing for. These days I almost exclusively play (and make stuff for) Dungeon Crawl Classics, and prior to that I was a big D&D guy for decades, which probably informs my 8.5 x 11 preferences.
Rulebooks, source books, adventure paths/campaigns and bestiaries, for me, need to be A4 and hardback.
Adventure modules should still be A4 but I can live with them being softback since they will get more flipping through them.
This more or less lines up with my preferences too.
It's funny; at first I was going to argue that my dislike for zine/digets/A5 size books had to do with not being able to read the titles on the spines. But of course that's true of any saddle-stitch book, and I have dozens of A4/8.5 x 11 saddle-stitched adventure modules, so I don't even know.
Yeah basically this. I have little inherent preference, but I absolutely loathe books that feel like they've been inappropriately compressed down to "digest" format - which in my experience is about 20-30% of "digest" format RPGs, and you never know which until you get them!
Hear, hear. And even when a creator provides screenshots or whatever in a crowdfunding campaign, it can be hard to tell. I have an adventure I bought that turned out to be a zine size book printed with a double column of text and it's pretty illegible (at least to my aging eyes). I wish I'd paid closer attention to the specs when I backed it.
I really like OSR books generally being digest size, as kind of a symbolic break with D&D. I can glance at my shelf and spot most of the indie games that way.
For actual practical use, though? It really doesn't matter. You can have a poorly designed traditional book just as easily (and arguably more often) than a digest-sized book.
And I love that the 3E third-party setting book Redhurst: Academy of Magic is laid out as an 11" x 8 1/2" book (yes, horizontally), emulating a college brochure.
You make a few great points here.
In theory I like the idea of having non-D&D books be differently sized so they're easy to spot on the shelf, but in practice that doesn't really affect me much because most of the stuff i have is 8.5 x 11, across decades of gaming.
Great call on the fact that poor design is poor design, and of course the larger volume of full-size books measn that proportionally, many more of them will have the shoddy design.
I've made one landscape style book, but unfortunately it's never been broadly vailable in print because it's through DM's Guild and at least when I put it out landscape format wasn't an option for PoD books there (not sure if it is now). It's too bad because I think it's a really cute book (it's about giant space hamsters, with copious references to 80s and 90s action and sci-fi movies) and my kid helped me write it. I made two print copies of it, one for my kid and one for my editor's, but that's been all.