First, the biggest change you can make in tires is careful selection of make and model. That can have up to a 60% change in RR depending on what you are changing from.
But you should be aware that rolling resistance, treadwear, and traction are a technology triangle. In other words to get improvemtns in one, you have to sacrifice one (or more) of the others. So careful selection of a tire will pay benefits.
Second, going larger in tire size (meaning more width or more aspect ratio or more rim diameter or any combination) has small positive affects on RR.
You can read more about this here:
Barry's Tire Tech
But the tire size you mentioned: 205/55R16 is incredibly popular and you will probably find that you will find your best combination in that size.