I was wrong, de disp in the previous post had to be in this way:
disp(['the area is:' num2str(white_pixel_count) 'pixels']);
Then, you have to add the code for the conversion, combining the two codes I gave you:
disp(['the area is:' num2str(objectAreaInCentemeters) 'cm2']);
Let's try this code with the phantom image so that you can check the steps of the code by hand, and understand what you are doing.