To get your perfect Ria Fashions size, lay any of your best fitting tops or blouses flat on a surface, and measure from one armhole (crease under armpit) across the chest to the other armhole. Multiply this by 2 to get your perfect Ria Fashions size. These sizes typically are a few inches more than your bust size. 

Reference images:

The size here would be 26 inches x 2 = 54 

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The size here would be 16 inches x 2 = 32

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