The size of the printing of the image has nothing to do with WordPress. It’s a browser/printer issue. If it’s a horizontal image, it needs to be printed landscape. The other option is that you could rotate the image, if you don’t want to tell viewers that they need to go into their printer settings and change the layout orientation to “Landscape” and not print in “Portrait” mode.

I’m not completely sure what you are asking. I wasn’t aware that you could print images directly from the WordPress gallery. If you wanted to allow readers to print an image, I’d recommend linking to the full size image in the blog post and then tell them to print it in the browser. It would shrink to the page size (unless of course, your image is too small to begin with).

I believe setting the $content_width is the same. I could be wrong, but I think you can get the same effect (for Default media to Full Size) by setting the medium image’s max width to your content width and keep the default set to medium. You can set that size in the admin (Tools>>Media).

thanks.