Hi! Not sure if I followed but hope this helps.
As there is no 'media library' like in WP, we need to open an image file on a computer to see. Might sound like an extra work as we can scale, use wrappers and set width for the image but if you do image optimization for SEO then handling the image on your computer is needed anyway and is not really an extra step. How wide the image needs to be on your page depends on the design. Foundry is based on the Bootstrap framework that gives us the 1200px wide frame to work, from that we can calculate the size that is needed, like half page wide or etc.
Foundry widths are essentially controlled by the Container stack. You can set a bit of text to be 600 px wide, or a 2 column bit of text to be 900 px wide and so on. So the width of your image area will be largely determined by how you set the Container stack. Easy enough to set it to be be 300, or 600, or 1600 pixels. Whatever you need.
There are a few stacks within Foundry that assume you want the content/image to take up the full width of the browser. In these cases you need to make a reasonable guess as to how large your image should be. In most of those cases I'll make the image to be used 1600 px wide. As @TapioMichael mentioned, optimizing your images first is always wise and this is done outside of RW.