Hi just registered.
I am having the same problem. I need an image 27x18cm and have re-sized in Paint.net no problem.
The problem is in the way Windows prints. Paint.net is fine but as soon as you send it to print, Windows only allows certain formats within the requested paper size.
For example if you state you are printing on A4 and you untick fit to frame it will still try and print using the whole A4 page and ignores the size you have set in Paint.net.
We need a program that will print on A4 to the exact size specified in Paint.net and that is what I am looking for. The Windows print system is very basic and totally useless for printing specific print sizes.
Unless someone knows different....