Classic Studio and the new Studio share the same endpoints - web experience, mobile app, etc. - and therefore the content specifications for how to make content look great in the member experience are the same across Classic Studio and the new Studio.
The image appearance optimization article content can now be found in this Firstup Knowledge Base article: How to Optimize Image Appearance in the Firstup Platform.
Feed card character count specifications can be found in this article: Content Specifications - Feed Card Character Limits.
Content specifications can be found in this article: Content Specifications - Image, GIF, and Video.