The crop tool in photoshop works like most tools it can be resized positioned set to a predetermined size etc.
A question first check and make sure under the view menu that you have the snap and snapto are unchecked.
I have included an image that I will try to explain how it works.
along the bar where I have circled in the blue color of which you can set, this will give you a preset crop size in this case what ever is inside your crop area will be the size of 800 x 600px at 300ppi.
the ones i circled in red are blank the sizing was blanked out simply by clicking on the clear button this will give you a freeformed size determed on the number of pixels that are inside the cropped area.
You can resize the area by clicking on and dragging the little sq handles at the corner and mid way on the sides.
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