For anyone still struggling, it seems noone has been quite resourceful enough to provide a nice answer to any of these questions.
1. Resize your image to the size you would like it to be. ex: 5cm x 5cm. Do that either by selecting it with the box selection tool and dragging a corner while holding shift to shrink it, or some other overcomplicated method with resizing your canvas.
2. With your resized image still selected, crop to the image (Image/Crop to selection). This will remove everything else not selected, so make sure you have everything selected that you with to print.
3. Change the size of your canvas to fit the size of paper you are using. (Image/Canvas Size...) from here it will open a window, be sure to select "By absolute size", then under print size, change to the dimensions of your paper (standard A4, or printer paper is 29.7cm X 42cm). Click ok.
4. Now reselect your image, move it out of the corner a bit to ensure it isn't cut off by the limits of your printer.
5. Print. as normal.
