@phoenix_eD Never heard about that setting before, I guess it's new ? Mangadex staff haven't had the time to make an announcement yet ? It's working perfectly though, thanks for the info !
edit: Oh wait, it doesn't seem to work on old chapters. I guess they only run that resizing script on chapters that have been uploaded since that option was added.
@MayhemMaster That's a bit of a broad question and I'm no expert, this being said :
at least half the resolution, in this case you could even divide it by 3. For reference the mangadex option mentioned earlier seems to resize pictures to 1000px wide. But just dividing the resolution by 2 means dividing the pixel count by 4, that's already a big saving.
maybe use png rather than jpeg. For "simple" pictures, png often offers better compression than jpeg. For reference, keeping the resolution, just a basic export in gimp of the first page here removed 43% of the file size (4.6 to 2.6). If you use jpeg, check lower and lower values on the quality slider until you start seeing some compression artifacts. For reference, I only started seeing some really bad stuff when exporting page 1 at 20 quality, but 30 quality was already down to 1.1MB.
All in all, if it's not a double page spread or colour page, you shouldn't need to go over 1MB.
There's ways to automate all of this so you don't have to worry about it, but it's going to depend a bit on the tools you're working with. Maybe drop by mangadex's discord, they'll surely have lots of advice.