CbxConverter is an easy tool to automate conversion of comic books for reading on KOReader. It's Windows-only, but there's a blurb on the website about it potentially working with Wine.
It allows you to mass convert to CBZ (since CBR files cannot be read on KOReader) and you can also resize the images: right click each job, select "Resize..." and enter an appropriate scaling under "Resize to" (use the Img width column as reference). If you're reading on an eReader, you can also add
to "before resize" to convert to grayscale, potentially reducing the filesize or improving legibility.
Make sure you change "output extension" to png or jpg, as by default it converts to WebP which KOReader cannot read.
I am in no way affiliated with the creators of this program. If you're reading this, thanks for this software.