tire inflation and tyre type affects economy more than size itself (within same size rim).
You currently have 205/65. You SHOULD have 205/55 (if you want something with same tire width).
Then there are tire makes that have smaller contact patch than others at the same air pressure (inflationary pressure).
Smaller contact patch = less rolling resistance = more economy = less braking power
If you are changing tires for greater fuel economy, DONT. The added expense isn't worth the 'potential' benefits.
For fuel economy, do a proper engine tune-up including a proper engine flush.