Slider sizes, content jumping - and serving different image sizes for mobile?

Hello -

I have a few questions regarding the slider functionality in the Dazzling theme:
I want to have the slider always show in a fixed aspect ratio (3.5:1). This gives me enough room to show nice pictures, and does not take up half the screen on a mobile device.

In my functions.php, I have set:
add_image_size( ‘dazzling-featured-slider’, 1920, 550, true ); //for slider
add_image_size( ‘dazzling-featured’, 750, 210, true ); //for posts

I have taken to uploading every picture for the slider as 3000x1000 and that works fine. However, when I upload something that is less than 1920 in width, it will enlarge the slider in height - which results in the content below jumping up and down with the size of the slider.

Is there a way to absolutely, once and forever, lock the size of the slider - so that I can upload pictures of whichever aspect ratio, without my content starting to skip?

Also, for these days, the slider size I picked appears big… What is, according to your experience, a good image size for the slider to use?

Last, but not least: When loading my homepage on a mobile device apparently I’m still loading the full-size images to the phone. Is there a way to make it serve smaller copies to mobile, ideally based on screen size?

(the website I am working on is

Thanks a lot for your help!