I'd go to less than 21". Preferably overall measure less than 20". The next carry-on restriction will certainly come and it will make things even stricter for sure.
Wheels may easily push it over the 20" limit or even the 21" limit. They also take space away. At that point the bag will have so little volume that you cannot really fill it up to make it so heavy that it would require wheels. On the other hand wheels add weight. If you get rid of the wheels, the bag might be so lightweight that you can actually carry it and don't even need wheels.
The rule of thumb for what you can comfortably carry is 10% of your body weight. That works pretty well. I weigh 73-74kg and I can carry 7-10kg easily. Over 10 it starts to get cumbersome.
If you use the roller bag to actually carry (piggy back) a second bag like a laptop bag, then it starts to become justifiable.
Still, spinners are fantastic for big heavy suitcases. For a carry-on they are overkill in the sense I explained above (reduced volume, added weight, more fragile). Most spinners also don't have outer pockets. But these pockets are highly desirable in a carry-on.
In a word, a small spinner is aberrant. Sorry.
If you absolutely need wheels, go for a lightweight two-wheeler. Check the "lightest bag" thread on FT.
Don't get me wrong, I totally dig my 32" Rimowa jumbo trolley. I can load that thing up with 70lb or more and maneuver it with two fingers on a smooth airport floor and with one hand on pavement. For a small case four wheels are just not needed.
If you do still want a small spinner the Rimowa polycarbonate ones are highly recommended and are among the rare ones that do have outside pockets. Not cheap, though. About twice what Samsonite would cost.
Till
