OP's real question is whether there is information about the actual dimensions of the under seat area for bag storage. Most aisle and window seats I have been in tend to have less actual area than the middle seat. OP asks about window seat area, which is compressed often due to the curvature of the fuselage.

I don't have a resource for the OP, but my wife has a small, square rolling suitcase that often fits. She will roll it to the gate but carry it by the handle when boarding. This works most every time. Using a soft sided bag would seem to be the best bet. Some have wheels and can be rolled.