Adjusting figure and axes size

I have a problem with setting the size of figures and axes objects in MATLAB. I have tried to set the size of a figure to precisely 20 by 20 centimeters for example by the following scripts

a = figure;
set(a, 'Units', 'centimeters')
set(a, 'Position', [0, 0, 20, 20])

But It makes a figure with different size! I have the same problem in setting the axes size. Also I wanted to know how can I accurately set its size in a way it does not change based on the screen resolution, and the size remains the same on different computer screens?

Thanks in advance Ramtin