Theme editing help

Unless the theme comes with an option for variable width, making the “white box” larger would probably mean modifying most other aspects of the theme as well.

If you are going to use the full size images and not link them, edit them to a consistent width that fits inside of the actual content area first. Alternatively, post your images as small or medium sized previews (thumbnails) that can float/wrap with your text, and have wordpress link them to the original file so that when people click on the image, it can be viewed in full size.