Hi Greg,

Thanks for sharing this question! At this time, we do not have a comprehensive guide for visual elements on the platform. This is something that we will prepare in the future. In the meantime, here are some links that may be helpful:

Bettermode Help Center | Appearance and Design
Bettermode Theme Guide - Learn how to customize your theme
What are all the recommended image sizes/dimensions?
Editing the Collections Menu

To answer a few of your points above:

Hero Banner: We do not have a recommended size for the hero banner due to its customization options. The minimum width of a hero banner should be at least 1,200 pixels. However, the height of the image will vary depending on the presets selected as well as the height option in the hero banner.
H1 Headers: H1 headers are not used for posts and blocks in Bettermode as it will impact the SEO. Currently, you can only format texts in H2, H3, body or paragraph text, blockquote, bulleted and numeric lists, and code blocks.
Collapsible Sidebar: You can adjust the Collections Menu in your sidebar settings (administration > Design Studio > Header and sidebar > Collections Menu > ⚙️) and under the View Style, you can select the Header option to disable the collapsing option.
Types of blocks: You can learn more about our building blocks here. Most blocks will have a title field. However, some blocks do not have them such as the Member Auth Card, HTML block, Composer, Section, etc since they are functional blocks.

A workaround that I often recommend is to insert a rich text block (with no padding or card) so the titles are consistent for every block needed. Here's a quick example:

I hope this helps! Please let us know if have any additional questions. :)

Documentation