I don't think you can constrain imrect(), rbbox(), or getrect() to work with fixed size rectangles and not allow the user to change the size of the rectangle. The workaround is to have two scroll bars, one for vertical movement and one for horizontal movement. The slider callbacks will calculate the new position of the box sides and draw it using plot(). Then the user clicks an OK button. In fact I do this for some of my apps.
The alternative is to use dragrect(). I haven't used this before. Here is the description from the help:
Examples
Drag a rectangle that is 50 pixels wide and 100 pixels in height.
waitforbuttonpress
point1 = get(gcf,'CurrentPoint') % button down detected
rect = [point1(1,1) point1(1,2) 50 100]
[r2] = dragrect(rect)
Tips
dragrect returns immediately if a mousebutton is not currently pressed. Use dragrect ina ButtonDownFcn, or from the command line in conjunctionwith waitforbuttonpress, to ensure that the mousebutton is down when dragrect is called. dragrect returnswhen you release the mouse button.
If the drag ends over a figure window, the positions of therectangles are returned in that figure's coordinate system. If thedrag ends over a part of the screen not contained within a figurewindow, the rectangles are returned in the coordinate system of thefigure over which the drag began.
Note: You cannot use normalized figure units with dragrect.
