I would like it if the 150 X 300 avatar size becomes the default size for all members. For Premium and contest winners they can have higher upload size and 150 X 350 avatar sizes. 150 X 200 is an outdated avatar size by 2018 standards as all the decent anime forums give 150 X 300 avatar dimensions or more by default. It's also annoying making these 150 X 200 avatars as they have a very odd dimension ratio and to use big avatars you have to crop them.
The Big avatar is also not permanent, once the Premium ends, your avatar gets downsized until you replace it again. This, in my opinion, makes it a meh bonus as you have to change your avatar or end up with a downsized avatar.
It's disappointing but you kind of have to deal with it I guess if there is no change.
With a new start, AB could incite many members to join by offering the 150 X 300 avatar size by default. There is potential if done, it will make many members happy.