Dawn theme need pixel length and width for any image

Hi,

I’m using the Dawn theme and I want to find out the pixel width and length of any space I want to place an image so I can create images to those specific sizes.
I created a slide show at numerous dimensions. 2000x900 looks good in the customize theme area but when I preview at full screen it messes up the dimensions of the file and cuts off alot of the image.
My first time on doing a website. Usually I’m designing images in specific pixel lengths and widths for social media or print.
How do I find the length and width I need to use in every section that has an image for me to insert?