It doesn’t really work the way your thinking. Each Kodi profile maintains a thumbnail cache and this cache is indexed by a local database that is seperate from the one that your sharing via MySQL and also cannot be shared. Because there is no way to actually keep the cached artwork in sync with the databases on each client even a shared MySQL database this means that one can’t maintain any kind of server level artwork settings or files. The source of the artwork is local files that are placed with the media (and named correctly) and if those are not present they are downloaded from whatever site your scraper is set to.
That setting your referring to for the artwork is the size the cached files are stored in. You likely don’t want to set it like that. See this thread…
The process you seem to be talking about in practice would simply be storing full size artwork with the files. This is fine but will have no difference to the end result over Kodi grabbing them online when it scrapes. Having local files will make it scrape faster but grabbing full resolution artwork online and full resolution local will still start with full resolution artwork that will be cached on each profile using whatever settings set for that profile.
If you want to have a more resilient setup on how you have your files on your server you might consider a media manager such as Tiny Media Manager that will make all local metadata that Kodi would then use when scraping. It also allows for easy customizations such as changing movie sets or custom titles.