How do you print photos at non-standard sizes?

Question

If you have a frame that is very small or at a non-standard size such as 2"x2" how do you edit your images so that you can print them at a standard size from a lab?

For example, if I want to print for a 2"x3" frame, the smallest print size at photobox is 4"x6". In this case I can shrink my image by 50% and surround the jpeg with a border then print it at 4"x6" so that I know when it is printed, I can cut out my photo and it will be at the right size.

Eg: transform this:

Into this:

This was simple in this case but how do you guys do it in more complicated situations? For example, what if I wanted to prints this image for a 6"x6" frame to be printed on 10"x8" paper?

These are my requirements for a solution:

1. It must be easy. Just because I'm good at maths, doesn't mean I should have to be. As Americans say, it should be "mom-proof".
2. Maintain picture quality. I expect the output image to be suitable for at least 300 dpi.
3. Not require high memory on the machine. Eg, simply multiplying the canvas size doesn't scale well, at least not on my old pc ;)

Answer 1

If you're using software like Photoshop, which works with a canvas, you can choose your canvas size when you create a new document. If I recall correctly, you can choose a size in pixels, picas, points, cm, and inches.

Answer 2

Size your image how you want it to be sized with correct dpi.

Use Photoshop to adjust the canvas size to the size paper you want with your image in the center and the extension color set to white.

Answer 3

I never found a simple (parent-proof) solution to the problem.

So I ended up writing an open-source website to do it: http://www.oddprints.com/custom

Answer 4

TdhCad can create a file that will produce an image of any specified size when printed on paper of a larger specified size. It's free and be found on Linux and MS Win stores.