Whether it's handles on the edges you can drag, or a marquee selection type thing you can... also drag. What I'm trying to say, if you'd LET ME FINISH! ...is that re-sizing the output image should (and shouldn't be) a drag!

Or wait wait...hear me out. What if there was an auto-resize button that would... wait for it... auto-resize based on the resolution and aspect of your original image! Then, once you catch your breath, you could adjust the margin between the uploaded image and the edge of the output image with one of those fancy sliders you like so much? Huh? Bet you're glad you read that all the way through!