With today’s cameras I would imagine you have plenty of resolution so as to avoid pixelization.
First, I would make sure the picture is 300dpi. It is done through a dialog box, Image->Image Size, I believe. When you change the dpi, the dimensions will change for the image, but you are not losing any pixels.
If the image is exactly what you want, then you can use the same dialog box to set the dimensions and you should be done.
If not, next I would turn on the rulers to use as a reference, since the passport photo needs to be a specific size. If you know how to drag down guidelines from the rulers do that (it’s easy, just click in the ruler and drag into the document. A line should appear, but it will not be part of the true picture).
Then you should shrink the picture to fit into the correct dimensions. You can shrink it either by using the transform function (Ctrl-T) or using Image->Image Size. You can crop the picture if it makes it look better. The pixelization should not be a problem.
Pauley