1. yes... fixed size marquee. Select marquee and in main bar click on style button, select fixed size, enter dimensions.
2. yes... selecting the section inversing and cutting the object by selecting that layer.
Thanks for your comment, but as far as I know the marquee tool is used for Selection but I need to Draw a line.Can you please let me know a little more about your second answer, please?
firstly: you can draw a line by clickink on the pen tool, look at the main bar, select the line tool. you can adjust weight etc.
oops... dunno what happened...
ok... lets say you have a rectangle with a star overlapping it (in separate layers). command click the rectangle thumbnail in the layers panel, then select the star layer, press command-shift-i to invert the selection and then press command x to cut it.