How do I set crop size?

Is it doing any size, or is it keeping the crop bounding box proportional?

If the former, then perhaps your crop tool isn't behaving properly and a reset of the preferences should restore the crop tool to its normal behavior. See the FAQ for instructions on how to do this (if you don't already know how to reset the prefs).

If the latter, then it's doing pretty much what it's supposed to do. What's happening is that when you select and then apply your crop, it will crop the image to that specific size. In some cases that's a good thing if you're going from a larger image to a smaller one. Sometimes, it's bad if you're going from a smaller size to a larger -- especially if you are also specifying a resolution with your crop, which will resample your image and can harm the overall crispness and quality of the picture.