What is the largest object on which the Yarkovsky effect has been observed?
According to Vokrouhlicky et al. 2015 Yarkovsky forces can be measured for small bodies with diameters up to 30-40 km. The largest object they have in their list of Yarkovsky detections is 4179 Toutatis with a diameter of (only) 2.8 km. I am not aware if Yarkovsky forces have been measured on anything larger than asteroids.