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
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
, 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.