You can't directly set the size of an image in CO. This is because CO is a non-destructive raw processor, and actually changing the dimensions of an image would be a destructive change. The only place you can specify a specific image size is in an export recipe.
However, all you need to do to get the image ready for printing is to crop the image to same aspect ratio as A4, and then it fit paper when you print. Note, when you crop may show the pixel size, but that is based on the currrently selected export recipe and is not actually changing the size of the image internally.