Why would I want the crop scissors on?
To get the exact size. Read the Help on "Cropping" in the listing of major functions. It sounds as though your image is not exactly in the 3:2 ratio of a 6x4 print.
The final paragraph in the Help says:
"Note that when "Crop to Size" is turned on (scissors button depressed), you have given Qimage permission to crop some of the image to force it to fill the entire frame." This does not affect the image, just the print.
I don't understand why for example if my paper is really 24" wide why I would have to set the printable area to 24.02?
Qimage reports exactly what the driver tells it. When you originally specified 24" and you saw a smaller figure in Qimage, that's the driver saying that's it, not Qimage. It seems that it is not possible to use the full 24" with your printer, even with borderless set. I assume must be a physical limitation of the printer.
I don't think you said which model printer you are using. Fred has a number of drivers installed so could possible check in detail.
Have you looked at the video? Using borderless will give you incorrect print sizes due to the expansion effect of the driver. You might try turning off expansion using the Qimage function. I've not got Q-SE loaded any more so you'll have to explore the menus to find it. Please watch the video!
Terry