Steve,
If you use the Crop Tool you can enter the measurements in the width (144 mm) and height (120 mm) boxes and you will have exactly those dimensions. If you use the Marquee Tool you can use the Fixed Size style. Also you may want to turn on the Rulers (View >Rulers).
Juergen
When you say 144 X 120, I assume you’re speaking in millimeters? (Sorry, but I flunked metric!)
You can designate a specific size when using either the Crop tool or the Rectangular marquee tool for cropping. When you select the tool, next look in the Option Bar, and you’ll find a "Style" area. Click on the little triangle in the lower right, and you can then choose "Normal" (the default), "Fixed Aspect Ratio", or "Fixed Size". Choose Fixed Size and type in the size you want, and the selection will be constrained to exactly those dimensions.
If you’re using the Crop tool, also pay attention to the resolution setting in the Option Bar and make sure it’s what you want. If, for example, you’ve been working on an image at 300ppi and the resolution in the Option Bar is set at 72ppi, your image will be automatically resampled at the same time it’s cropped. This can be good if you realize what’s happening, but some people have been surprised to see their pictures turn out less crisp and clear than expected.
Edit: Well, Juergen beat me to it, but I added something about resolution! 🙂
If you have your Rulers opted for (Control/Command R) you can Right click on the ruler and opt for any number of measurement units, including millimeters and pixels to go by. If you drag from the upper left of the rulers you can establish the 0/0 point where ever you wish to drop it.
Bob