It really depends on what photoshop you have, but if it's CS, CS2, or CS3, select the crop tool and on the tool bar above, there are boxes for you to insert values. just backspace/delete the numbers in both the H and W boxes and you can freely size the rectangular crop.
If you wanted a freestyle crop such as cropping a picture into a star shape, circle, or a random shape, one way is to hit the hot key "q" which does something w/ the canvas which i can't remember what it is called.
After that, select the brush tool and then brush the area you want to select. Wherever you brush should be highlighted a transparent red. Either that, or the whole picture is red when you hit "q" and your brush wipes off the transparent red wherever you want to crop.
It doesn't matter whether it's all red or all normal when you hit q, the important thing is that when you use the brush tool to paint what you want to crop, it should highlight one or the other.
After you are done painting what you want to crop, hit the hot key "i" to invert the picture, and then hit the hot key "q" to exit the canvas thingy and you should then be left with a selected part of the picture. Hit delete to erase what is selected and you are left with what you wanted to crop.
If that crops the opposite things of what you wanted to crop, just undo, hot key "q", hot key "i", hot key "q" to exit, and hit delete.
I'm not very good w/ the terminology of photoshop, but I hope this helps.