[paw broon]

DownunderDan has uploaded another issue of the Australian Captain Atom.  The book is in landscape format and he mentions how bemusing this format is. But it was widespread in Spain, Italy, France, Netherlands (although with much fewer titles). Occasionally used in the UK mostly for reprints of newspaper strips, for which landscape is the best format.
But Australian publishers not only experimented with the standard landscape shape as in Dan's Captain Atom, they also had for a short time, a calendar format where the cover is portrait with staples at the top and you turn the book on its side to read it in landscape. Early issues of The Phantom were like that and there is a rare variation where it is very calendar like and is a portrait comic but with staples at the top and the reader flips up the pages.
French landscape comics featured a range of sizes and there didn't seem to be a standard. 
It could be that publishers printed in whatever size or format the printer could do - whether paper size or machine capability.  Comics historians have written that the British pocket libraries were in that small, pocketable size because at the beginning, the printers could only offer that size.
Oddly, Australian reprints of Dell westerns appear in both pocket portrait size and "normal" comic portrait.
Italy experimented with a pocket size square format.
American comics and Japanese manga are unusual in as much as they conform to a particular size and shape for the most part, but that is changing especially in manga, whereas British, French, Italian, Spanish comics, appear in a multitude of different sizes and shapes incl. those landscape strisce/liliputs/piccolos.  One or two tiers to a page and even here, there are different sizes. The Dutch versions are much larger books with the Belgian reprints being slightly larger again
(If anyone fancies collecting Erik de Noorman - a beautiful strip - the Belgian reprints can be cheaper to buy than the Dutch books.  The quality isn't quite as good in the Belgian versions.)
Those pocket libraries are a deliberate choice of the publishers.  They fit in your pocket. 

[crashryan]

American comics have always had the same general proportions though page sizes have shrunk over the years. The newspaper strip reprints that predated comic books were roughly 8-1/2 x 11 inches, the size of periodicals like Life and Judge. When comic books came along, though there were oddities like the oversized Jumbo, most measured roughly 7x10 inches. That format remained the standard for years. When publishers reduced page size to save money page width shrank more than height. I believe that the page size was related to the size of the raw sheet that was fed into the press and later folded and trimmed to form finished pages.

I've seen a few comics in odd formats. Some giveaways like Dell's March of Comics were printed in landscape format. St John's It Rhymes with Lust was in digest size, around 5-1/2 x 8-1/4 in. This was the format pulp magazines had adopted when they downsized in the 1940s. I have a set of four small landscape color comics reprinting a Terry and the Pirates episode that were included as premiums in Canada Dry ginger ale cartons. I know some of the American comics on CB+ were in unusual sizes but it's hard to tell which ones by looking at them on a screen.

[Drahken]

I still have a large selection of newspaper reprint comics in US paperback format from when I was a kid. Single panel comics worked well enough in that format (at least the dailies, not so much the sunday strips), but multi-panel strips always had to be broken up and rearranged oddly (usually in a vertical & staggered arrangement).

[crashryan]

I still have a large selection of newspaper reprint comics in US paperback format from when I was a kid. Single panel comics worked well enough in that format (at least the dailies, not so much the sunday strips), but multi-panel strips always had to be broken up and rearranged oddly (usually in a vertical & staggered arrangement).

Yeah, the American paperback book format was a poor one for comics reprints. I had the EC and Tower paperbacks. The EC s-f reprint, Tales of the Incredible, was my introduction to EC comics. The format only worked if the original comic stuck to a strict 3-row layout. 2 regular panels or one wide one fit onto a paperback page, though you had to turn the book sideways to read them. Tall panels, splash pages, and other variations were shrunk or blown up and stuck onto the page leaving big empty spaces. The trade paperback format they use now works much better.

[paw broon]

Some of the Mad paperbacks had sideways strips. The British paperbacks with reprints of Batman, Mandrake, Superman and others were annoying with the original work cut up and pages awkwardly laid out.  Frustrating.  Despite that, they were collected as curiosities.
Spanish publishers produced lovely hardback landscape books of newspaper strips.  Only way for me to get Rip Kirby stories.