When you select the Rectangular Marquee tool, one of the Style options is Fixed Size (just below the Menu bar). You can use any of the units Photshop recognizes; e.g. in or px. You can move the selection accurately with the arrow keys on your keyboard once you make it. I hope you didn't spend too much time trying to eyeball it.
The last post about cropping got me thinking about this question, I didn't want hijack that thread so I'm starting a new one.
Many times I would like to make a rectangular selection to the height and width that I specify. As a example, lets say I want a 8x12 rectangular selection box, is there a way to do this in CS5?
The way I'm doing that now is to watch the ruler as I pull out the selection, but trying to hit 8x12 this way is, well, hit or miss. No matter how close I try, I never get exactly 8x12 this way.
I've tried using the crop tool, but its the same thing, you still have to pull out the selection.