A few years ago I was looking through lists of new SFF novel releases and came across one I've been trying to find again since. It PROBABLY appeared either on Locus or SFSite's "books out this week/month" lists at sometime since 2010, since I read those frequently back then, and would have been published in English.
The description spoke of a fantasy world with what to me seemed like an interesting gimmick... the world seemed extremely constrained in dimensions, and notably longer than it was wide. I can't recall the exact dimensions, but in shape I pictured something like an aircraft carrier... that is, mostly but not precisely rectangular. To my recollection, it wasn't isolated by natural barriers like a sea or mountains, but rather either a mystical force field or just nothingness outside (maybe even a wraparound, but I don't think so).
It's also notably small, at least on a planetary scale... it's not a case where it's the landmass of a whole planet that happens to have a weird shape, it's a case where it's one small 'strip' of land (probably at the most the size of a small country or two, one of the reasons it stuck in my mind was that it caused me to think about what the smallest land area that could develop a sustainable functional medieval-level civilization with no outside input might be) and being, as far as they know, the only thing in the universe. I believe the description (or perhaps an author interview I may have followed off a link) specifically stated that it was so-and-so number of miles long by so-and-so-number of miles wide and both were reasonably small numbers, I just can't remember how much.
I can't vouch for whether this was always the case or a relatively new development (but I think at least the main characters think it was always the case, and I don't think it was a fragment identifiable as part of Earth). I also unfortunately can't remember any characters or plot mentioned clearly (MAYBE a princess main character, and POSSIBLY the border being called something like the Veil but my mind may have pulled that from somewhere else). My memory also has the sense of this being new at the time and a first novel, both in the sense of not being (yet) part of a long fantasy series but also the first novel by an author, though I can't be certain. I thought the gimmick was interesting enough that I mentally sorted it into the 'well, maybe if I hear some really good things in the years to come' category. I didn't, I've never heard about it again, so I guess it wasn't something that was a huge success. I don't even particularly want to read it, my brain just really wants to figure out what book it was.
Now, generally fantasy doesn't capture my imagination as much as science fiction in prose form, and in fact usually in new release lists I just skip over anything labelled fantasy. For this one to have caught my attention, probably either: a) the nature of the universe being very tiny with nothing outside it had to be prominent in the very brief description new release sites often give (I think this was the case), b) the description/title had to look like science fiction and only closer inspection revealed it to be fantasy-but-with-an-interesting-gimmick (possible but I don't think so), c) the author had to be someone familiar to me in science fiction circles (but I really don't think so, I believe it was a first novel), or d) it just had really a cool title and/or cover that got me to look deeper into the book (which is the least useful, I know).
I know it is NOT The Hundred Thousand Kingdoms by N.K. Jemisin, because the plots aren't similar, but for a long time that name got associated with it in my head (even now I think of it as 'The book that's NOT The Hundred Thousand Kingdoms'), so I think they may have come out at roughly the same time, which would ne 2010, but it could also be the that a new printing or sequel came out at that time or they were directly compared in some way or maybe the titles simply had a similar sound (the Hundred Thousand Kingdoms being a book I also looked into because the title really sounded cool).