Why do you need to control the size of the frame exactly? (Not saying there is no reason to want such control, but if you say what that reason is, maybe there're some other means to achieve what you are after.)
Did you place the rectangle to the exact coordinates? If not and the rectangle is there to indicate the area of interest just roughly, resizing the frame visually should be fine. If yes, then, since you are entering exact coordinates already, you might try computing and setting the size of the frame in millimeters: take the absolute scale from the view, count the number of millimeters by X and Y in the data from the coordinates of the rectangle, apply the scale (eg, if the scale is 1:50,000, divide by 50,000) -> the result is the size of the frame.