Automatically maximize a figure

This worked for me:

figure('units','normalized','outerposition',[0 0 1 1])

or for current figure:

set(gcf,'units','normalized','outerposition',[0 0 1 1])

I have also used MAXIMIZE function on FileExchange that uses java. This is true maximization.

For an actual Maximize (exactly like clicking the maximize button in the UI of OS X and Windows) You may try the following which calls a hidden Java handle

figure;
pause(0.00001);
frame_h = get(handle(gcf),'JavaFrame');
set(frame_h,'Maximized',1);

The pause(n) is essential as the above reaches out of the Matlab scape and is situated on a separate Java thread. Set n to any value and check the results. The faster the computer is at the time of execution the smaller n can be.

Full "documentation" can be found here

As of R2018a, figure as well as uifigure objects contain a property called WindowState. This is set to 'normal' by default, but setting it to 'maximized' gives the desired result.

In conclusion:

hFig.WindowState = 'maximized'; % Requires R2018a

Furthermore, as mentioned in Unknown123's comments:

1. Making figures maximized by default is possible using:

   set(groot, 'defaultFigureWindowState', 'maximized');
   

2. Maximizing all open figures is possible using:

   set(get(groot, 'Children'), 'WindowState', 'maximized');
   

3. More information about 'WindowState' as well as other properties controlling figure appearance can be found in this documentation page.

Finally, to address your original problem - if you want to export the contents of figures to images without having to worry about the results being too small - I would highly recommend the export_fig utility.