Perhaps you’ve already found a way to resolve this issue. My site build has been a rather protracted process – I only recently addressed this issue with a WP designer, so apologies for the very late response.
What he told me is that WP will automatically re-size images on your pages to fit the viewing platform. You have to make sure that for a given image group/class (images that you want to appear as the same size), you have to edit them before uploading them to your media library (or once they are there) so that they have the same aspect ratio and pixel dimensions. The edit media window only pops up when you upload images for your posts (blog page). You can change the default sizes for thumbnails, medium, & large images through your dashboard >Settings>Media.
Its seems like if you want to have the same attachment display settings option for images you load other than those for your posts, and/or if you want to add new default size(s) for your post image settings, then you’ll need to find a WP guru who can write some custom code for you, or perhaps work with your theme developer if your theme is subscription based.
I hope that is somewhat helpful! Best of luck & Cheers!
