Unable to edit size images

Hello, I am testing/creating my first blog post (my site Is not yet live) when I add a large feature image it looks fine. However when I then go on to add several different images in the rest of the post and edit the sizing so they’re all of similar proportions, I hit preview and randomly a few of the images will appear drastically larger than the others.. so I go back to edit it again and when I do this, now another image appears larger than the rest. I can’t seem to find a solution to make ALL of the images (minus the feature image) of a nice equal size, so they do not look disproportioned and blurry? I have no idea why this simple task of adding images to a blog post is so difficult?

Urgent response required, thank you. I’m using a custom Opaque theme.