TL; DR; PNG resized to 1080 pixels on the short edge.
Just recently started dabbling with Pixelfed, but I couldn't find the best upload settings anywhere in the documentation, bar the 15MB upload limit. The server documentation states a quality percentage to use and boolean switches whether to resize and/or optimize images using aforementioned quality.
Turns out: images are by default resized to fit 1080 pixels on the short edge and are re-encoded using https://image.intervention.io/v2/api/encode set to the server's chosen quality percentage (which usually isn't advertised for instances but is 80 by default). Luckily, PNG is accepted nearly everywhere and its compression is lossless.
Those PNGs are way bigger than actual full resolution WebP's with a quality over 80% for my 24MP RAW exports.
Most resized and optimized images tend to be smaller than 1MB, though, whereas the allowed PNG's are just shy of 3MB.
A full-res 24MP test image on 85% quality WebP dives well under that 3MB. Restricting to something more modern say 4K monitor resolution at 85% WebP would be well within current optimized file size ranges of about 1MB from my experiments with Darktable exports.
that's a good explanation, thank you very much ๐