3:2 is a convenient ratio, as you should be able to print on 6:4 paper
Depending on the picture, you may be able to crop it to specification, providing there is no important information which will be discarded by this action.
Image>Resize>image size allows one to resize, but again if you uncheck "constrain proportions" there will be some distortion.
Depending on what you are trying to do, you can make your own canvas via File>new>blank file, and then go to File>place, point to your picture file, and it will come in on a new "smart" layer, which can be manipulated with the corner handles. If the canvas is not filled entirely, you can fill in the empty areas with color or a gradient. Go to File>new>blank file and under background content select "transparent'" Go to File>Place and place, Activate the paintbucket tool and fill the transparent areas with color.