Originally Posted by johndoe123
What's the largest piece of wheeled carry on luggage that I can take on any US based carrier?
You'll have to check with the specific airline (web sites help). It can also depend on which class of service you are flying in, and whether it is domestic or international.
For practical purposes, also note that overhead and underseat storage spaces vary in size between different airplanes and different seats (e.g. some WN 737 planes have overhead bins large enough for "22-inch" rollaboards to go in perpendicular to the aisle, allowing more of them in the bin, but others are just slightly too small). So soft squishy carryon bags can be useful to fit in spaces that are just slightly too small in one dimension, if the contents are not fragile enough to be damaged by being squished.
Also note that if you are in the last boarding group in economy class, you may find the overheads filled up by previously boarded passengers, so you may have to gate check your carryon if it won't fit under the seat.