Hi @agent99,
I hope you are well today and thank you for your questions.
You have too many questions and some of them are confusing so just let me tell you information about Sparkling theme so that you can understand it better.
The theme doesn’t change or set anything in the Media settings. The theme just contains two predefined images sizes. For the post featured image the size is 750px X 410px and for the popular posts widget images it is 60px X 60px.
Please note these sizes only apply to post featured images and popular posts widget images not anything else like Gallery images.
If you want to use post featured images and popular posts widget images different in size than the stated above then you can use the custom plugin solution shared in the following topic.
https://colorlibsupport.com/t/how-can-i-stop-resizing-featured-images/
The theme sets content width to 648 for normal pages having sidebar and for full width pages without sidebar it is 1008.
The width of Jetpack gallery is controlled by content width that is 648 and you can change this content width using the following solutions.
Best Regards,
Movin