You can easily change the size of avatars in messages on just about any style I'd say. XF uses specific sizes to load avatars, by default messages use m (medium). You can edit the template to l (large) to make this larger. There is some additional CSS you might need to help out if you don't increase the message user info width (style properties -> messages).
Our styles have an option to change the avatar in messages without you having to edit templates and that custom CSS code I mentioned.