Book or letter template for EPUB export

Of those two page sizes, I don’t think it much matters.

For most books (those with flowing formats), EPUB readers will flow the pages together, as you’re probably well aware. Change the displayed font size on an iPad, and you’ll see the whole flowing format book repaginate itself, for instance. With a fixed format, you own the whole pagination process, and the reader can solely zoom and pan their display to view whatever part of the fixed page layout they seek to read.

How to decide? You’re going to want to export and look at the results (whether flowing or fixed), and quite possibly reviewing the resulting EPUB when viewed in a few different EPUB readers and/or in EPUB reader tools. This to see whether the images used fit in the expected displays, and for rendering problems.

EPUB readers with smaller EPUB reader displays and fixed EPUB files will be zooming, and an iPhone Books display (for instance) may or may not be easily readable with a larger page size and with images sized and placed to fit that page size.

Using the Image Gallery exports to EPUB as a single image with right and left arrows to access other images that were added to the gallery. If you do not prefer this form of presentation, then stick with individual images from the Photos menu item in the Media Browser, or drag and drop them separately into Pages. You may want their Arrangement tab -> Text Wrap set to None for more control of the individual image positioning in your document.

Exporting a document to PDF with an Image Gallery only reveals the first image in the PDF with no controls as are found in the EPUB behavior.

I don’t think it much matters.

Try it.

Create a big page or three, fill it with Lorem ipsum text and a few images approximating your design thoughts, try it, and see.

Run some tests across the EPUB readers that you have access to, and preferably testing some of those that expect your readers to use, and see how well it works.

Try again with the other sizes.

These test pages both to check your page layout, and to check the rendering. From what I’ve met of it, EPUB rendering isn’t always as consistent across EPUB readers as any of us might want. (See links below for some tools.)

PDF will have the same issues with sizing as does fixed EPUB, with pan-and-zoom required to read the contents on small screens.

HTML goes the other way with this, with the HTML readers (browsers, and the browser users’ preferences) in control of more of the rendering and display, akin to EPUB flowing.

As mentioned above, I don’t expect an appreciable difference between viewing 8.5” x 11” or viewing 10.67” x 14.22”, particularly when viewing on a smaller display. It’s all going to need to be panned and zoomed. Or very young and very sharp eyes. But only you can tell if your fixed or flowing layouts are what you want, across your target devices.

Related pre-flight tools:

https://ebookflightdeck.com/

http://validator.idpf.org/

https://github.com/w3c/epubcheck

ps: That particular “odd” page size you’re looking at happens to be the size of a 1024 x 768 image at 72 dots per inch (dpi): 1024 / 72 dpi by 768 / 72 dpi is 14.22” x 10.67” in size.  72 dpi is an old printing and CRT and lower-resolution LCD display rendering. That page size would render nicely on an old CRT or a really low-res LCD without needing any scaling. For comparison, current typical printing runs at ~300 dpi, and variously higher, while the iMac 24” M1 has a resolution of 4480 by 2520 resolution with 218 pixels (dots) per inch. iPad Pro 12.9” is 2732 by 2048 resolution at 264 pixels (dots) per inch. This also all feeds into the image resolutions you’re embedding too, as a 72 dpi image might not look the way you want on a 264 ppi/dpi display.