According to their twitter MZ is getting an update soon which will allow for 32x32 tilesets!
https://twitter.com/RPGMakerSeries/status/1493064956937265156?
Viewing post in PIPOYA FREE RPG Character Sprites 32x32 comments
I look forward to seeing that feature implemented!
As for the size of the character sprites, I don't think there were any size restrictions even today, so I think 32x32 could have been used. I was concerned that if they were to be used with 48x48 size ones, they would be uncomfortable and we would need to standardize the size of one or the other.