Problem:

When I try to add the custom image size I get an error message: Zero size image string passed

Solution:

It is a compatibility issue, please deactivate all other plugins, and switch to WordPress default theme 2021, and test again

Relevant Documentation:

Problem:

How to add custom content to Toolset Maps pop up.

Solution:

In the section where you can add the pop up content, you can add shortcodes.

So the best way is to create a content template (Toolset > Content Template) and design whatever content you want to show.

Then retrieve that content template using the shortcode below in the pop up content:

Change the name of the content template with the one that you create.

Relevant Documentation:

https://toolset.com/course-lesson/displaying-a-list-of-posts-on-a-map/#add-marker-pop-up-content-and-hover-hints