Khanh - if you are using GUIDE, then open the property inspector for the figure/GUI, and change the Units property to normalized, and the Position property to [0 0 1 1]. The first two elements of the position vector correspond to the x and y coordinate of the top-left corner of the figure/GUI; the third and fourth elements correspond to the width and height of the GUI respectively. Setting these values to one (and since units are normalized) allows you to set the GUI size to the screen size.
If you are not using GUIDE, then you can set the figure to be the full screen size as follows
h = figure;
set(h,'Units','normalized','Position',[0 0 1 1]);
