Fed - according to http://www.mathworks.com/matlabcentral/answers/102219-how-do-i-make-a-figure-full-screen-programmatically-in-matlab, you could create your full screen figure as
hFig = figure('units','normalized','outerposition',[0 0 1 1]);
So if you have displayed your image using either imshow or image as
image(myimg)
then you can change the size as
% get the figure and axes handles
hFig = gcf;
hAx = gca;
% set the figure to full screen
set(hFig,'units','normalized','outerposition',[0 0 1 1]);
% set the axes to full screen
set(hAx,'Unit','normalized','Position',[0 0 1 1]);
% hide the toolbar
set(hFig,'menubar','none')
% to hide the title
set(hFig,'NumberTitle','off');
Try the above and see what happens!
