"Video/Player Avatar Size" settings are not saved between Roll20 sessions: because of this, is needed to configure it each time you launch Roll20.
For instance, if you set it to "Names Only" in "My Settings" (this is what I always do, BTW), next time you launch Roll20 the "Video/Player Avatar Size" size will appear set to "Regular" again.
I think that it would be suitable to *keep* the "Video/Player Avatar Size" specified by the user in "My Settings", as it happens with other parameters (i.e., "Display Name").