If I were doing it, I would not uncheck that little box as then the photo will be distorted in one dimension. What kind of application are you sending the photo to? Does it go in a document, or is it to be printed out as a small image? If it is going to be printed, what kind of resolution does the printer have?
What I would do is figure out the proportions and then crop the photo to have the right proportional dimensions. If you are going to print it out, you can control the image size in the print setup. If you are going to use it in a document, just set the scale to be the right size without loosing resolution. If you are going to be using it on a webpage or other on-screen application then save it with less resolution as a gif or jpg in the size you actually want.
I hope you didn't already try unchecking the box as suggested.
Yrs,
MoonHare
MoonHare@starband.net