Not sure what you mean when you say “control” them? The ones provided by Plex already have some restrictions so that the ones you can pick are not insanely broken.
You however can upload anything you want, the desired dimensions are quite varied as some logos will be wide while others are taller and narrower. Best is to just upload some and see which look best.
This is already implemented, see the forum post: Customizing Logos and Square Art for your Personal Media
