To measure the width, take a  measuring tape and measure between the left and right edge of the inside of the window frame (all done at the center of the window).  The inside of the window frame is determined by where a headrail bracket would be installed.  This is always done closest to the window sash.  For windows that open and close, the window sash is a piece of wood at the top of the bottom portion of the window that goes up and down.   Imagine a bracket installed touching the window sash, and that is where you would lay your measuring tape.

To measure the height, take a measuring tape and measure from the inside top of the frame down to the top of the lower window sill

If you need a return on the valence, measure out 2 inches from the inside mount.  Use that as your point of reference and measure back to the window frame where  a return would touch.  Standard returns on most windows are 1 1/2 to 1 5/8 inches in length.