Decreasing rolling inertia is most useful in stop-and-go traffic. It will have less impact on highway fuel economy. This is because with a larger rotating inertia, you need more energy to spin the wheel up to a certain speed and then you have to waste more energy (via the brakes) to slow it down again.
Reducing rotating inertia is done two ways: Reducing mass and moving as much mass as possible closer to the hub. Going with bigger wheels pushes more mass out away from the hub but this can be offset by simply having less mass.
If you want to upgrade to larger wheels without hurting your fuel economy, get the lightest wheels you can afford.
For reference: I have an old, cheap set of 18x8 alloys with snow tires on them and the wheels themselves weigh 25 lbs each. When using my highly calibrated "grunt intensity meter" (how much I grunt when installing/removing), I would say that they are about the same weight as the OEM 18" wheel/tire combo from my CX-3. They weight a subjectively YUGE amount more than the 17x8 Enkie Racing RPF1 wheels (~16 lbs) with extra wide summer tires for my RX-8. My wife can easily mount my race wheels/tires to my RX-8 but struggles to mount the OEM wheels/tires to either my '8 or '3.
I would guess (and this is a very rough estimate) that if you can get new wheels that weigh less than 20 lbs, you probably won't see much difference in fuel economy due to rotating inertia (tire choice will also have an impact). If you can keep the weight around 15 lbs for the wheel, you might even see an improvement.
