Cover thumbnail size

[JSaterdalen]

Describe the bug/problem
I enabled cover thumbnail generation to fix performance issues with large covers on my Kobo. Unfortunately, the covers are very pixelated and illegible when displayed on the Kobo sleep screen.

To Reproduce

1. enable "Generate Book Cover Thumbnails" in Calibre-Web
2. sync books to Kobo
3. open one of the synced books, then put the Kobo to sleep

Expected behavior
The thumbnail covers should be smaller than the full size covers, but not so small that they are unusable on the sleep screen.

Screenshots

Environment (please complete the following information):

• OS: Ubuntu Server 22.04
• Python version: 3.8.10
• Calibre-Web version: 0.6.19
• Docker container: LinuxServer