impact of wheel size versus wheel cover
 

I am wondering whether wheel size or wheel cover (with a smooth design, like a moon) has the greatest impact on fuel economy, and by what percentage for each? Is there any research data on this?

Wheel size is three things: diameter, width and offset.

Which are you considering.

195/50R19 versus 195/60R17

Assuming:
195/50R19 -- 26.7x7.7
195/60R17 -- 26.2x7.7

I leave the [fat tire-small rim]/[thin tire-big rim] debate to others.

Diameter is given, width is the same. The offset will affect how the tire sets in the wheelwell. Flush seems best, but poster Cd wonders about examples like Blowfish and other racers.

https://i.pinimg.com/736x/c7/ce/b8/c...cde2487821.jpg
i.pinimg.com/736x/c7/ce/b8/c7ceb80bd139c38ef9218bcde2487821.jpg

I would put the rim width equal to or greater than the tread width.

https://i.imgur.com/Cdh4U9B.jpg
i.imgur.com/Cdh4U9B.jpg

The wheel disk overlaps the tire bead to give the smoothest possible profile.

'MOON'
 

The MOON's ancestors were on World War-I biplanes.
And before WW-II, Bob Rufi was using war surplus aircraft wheel covers on his home-built land speed record dry lakes racer.
In Fig. 8.13, page-128, of AERODYNAMIC DRAG, by Sighard Hoerner, 1951, he depicts the lowest-drag aviation 'Streamline Wheel', originally presented by Becker & Leonard, in NACA Technical Report # 750, showing Goodyear's Cd 0.12 'oval' unit ( a spitting-image of the front tires/MOONs, of freebeard's Bonneville Tall-Boy race car that he's shown ).
In 1963, Walter Korff, Chief Aerodynamicist for Lockheed Aircraft Co., and designer of the Summers Brothers' 'Goldenrod' LSR streamliner, advocated for the MOON cover design, holding it up as the standard-bearer for low-drag wheel covers.
Paris Dressmakers hate them!

Under most circumstances, the larger diameter of the 195/50R19 would be better.