i would keep it stored where it already is, since some users might want to have different thumbnails settings for different files.
but i’ve noticed one thing:
there is shortcut for showing thumbnails in timeline, but there’s no apparent change unless the thumbnail size is more than 1. but 1 is its starting value.
i’d say it’s confusing to hit show thumbnails shortcut and not see any of them, so it seems reasonable that thumbnail size should start at 2.
of course that value could be a separate slider in preferences/timeline just like default starting frame - i can imagine that being useful on smaller screens.