Ran into the same issue as OP, eventually found a workaround, so I'll document it here in case someone ends up here later via Google.
Issue description: In device sync settings, all other settings & options are updated and saved correctly, except for the checkbox and number for "resize artwork to:" under "Embed artwork inthe music file" — whenever I restarted MusicBee or even switched from the device settings view to another MB view and back, "resize artwork to:" would reset to ticked and the number value would reset to 200. Other settings, such as selecting a different artwork storage method, are applied and saved correctly.
Version info: Win 10, MusicBee 3.5.8366 Installer, also tried the portable version, as well as a fresh installation in a non-Windows folder and running each of the aforementioned as Administrator, same result with all of them.
Workaround: Looked up the settings files (MusicBeeSettings.ini and MusicBee3Settings.ini under %AppData%\Roaming\MusicBee, unsure which one did the trick) and edited the Width and Height values under <Artwork Size> nodes from 200 to a higher value. There will be multiple <Artwork Size> nodes in the settings files if you have multiple devices saved, so you'll need to either figure out which one is the affected device, or just edit all of them (I did the latter).
If there is any other info I can provide for bugfixing purposes, I'm happy to do so, since this seems to be quite a rare issue.
EDIT: Oh yeah, and setting Artwork Size to zero disables the artwork resizing, if you'd like the artwork to just stay at the size it is in your library.