As of right now, covers are imported at 60x60 for all accounts.
Of course, you can modify the size individually in each account, by editing the CSS, but unless you also change the import size, the covers will become blurry when enlarged.
And if you modify the (global) size at which covers are imported, this will in turn apply this modification to all the places where the covers show (Widgets/Start Page/etc.) for all the users (even if they want it or not)