See my attached function (thanks to Yair Altman) for the best way. Make sure all the 'units' properties of all the controls on your gui are set to 'normalized'.
Or else make it the screen size, but without maximizing, by doing this:
% Enlarge figure to full screen, but not actually officially maximized.
set(gcf, 'Units', 'Normalized', 'Outerposition', [0, 0, 1, 1]);
