Have you checked out the wheel/hub combinations on
Servocity? They have this
set screw hub in a 6 mm version and it's only 9.5 mm wide. Also, the shaft can go all the way through the hub so the width is almost inconsequential.
You can pair it with these
wheels which come in various sizes. The smallest is 50.8 mm diameter. They have a large hole in the center and the hub can be mounted to either side, so they could be mounted almost flush with the motor giving lots of flexibility to meet your requirements. They are fairly skinny though at only 5.8 mm wide. If you feel you need a wider tire you could potentially bolt two of these together to double your width.
Another option for a heavier duty wheel is these
skate wheels but they exceed your limits being 75.6 mm diameter and 23.9 mm wide. They are also more expensive and require this
drive wheel adaptor to use with the previous hub, or you can forego the hub and mount them directly using this
drive wheel adaptor for 1/4 inch shaft (which is 6.35 mm).
All the previously mentioned parts are also available from
Sparkfun.
Pololu also offers a wheel/hub combination that would fit your requirements. They have this
wheel which comes in various colors (black, blue, red, white and yellow) and is 56 mm diameter has a tread approx 6-8 mm wide and is 11 mm wide at the shaft.
It works with this
hub for 6 mm shafts with 4-40 threaded screw holes, or this
hub with M3 screw holes.
Lynxmotion offers these
neoprene tires that are slightly smaller in diameter than your specs at 44.5 mm in diameter and 12.7 mm wide. They pair with this
hub for 6 mm shafts.
They also have a slightly
larger tire at 54.1 mm diameter but it is wider than your requirements at 20.32 mm. It would use the same hub as above.
In addition they have 19.1 mm wide tires in
57.2,
63.5, and
76.2 mm diameters that all use the same hub as above.
I know none of these meet your specs exactly, but they should be close enough to fit your needs.