Measuring
photo #7 on my laptop's screen with a set of digital calipers, I get 2.19mm for the receiver width/height, and 20.70mm for the height of the wheels (from the viewing angle, you can't use wheel width, since it projects on the image as oval)
Assuming the wheels are 19" (the sidewalls look to high for 21"), I get 19"/20.70mm, or 0.9178 inch/mm photo scale. 0.9178 in/mm * 2.19mm = 2.01"
Assuming the wheels are 21", I get 21"/20.70mm, or 1.0145 inch/mm photo scale. 1.0145 in/mm * 2.19mm = 2.22"
Class III and IV receivers are 2", class V are 2.5" (there is no 2.25" receiver). Plus, if you are trying to claim "better towing capacity than anything in it's class" (or whatever the quote was), you aren't installing a dinky 1.25" class II receiver (max 3500lb trailer weight).
TL;DR - Conclusion - My math says it's a 2" receiver.