You can use simple trigonometry to calculate the width of the FOV for a specific distance from the camera.
Your question calls out the 120x120 deg FOV of WFOV mode. There is also the 75x65 deg FOV of NFOV mode. You can find the full specifications here
Specifically looking at the 120 def FOV. You can split the FOV into 2x 60 deg (angle between acute and hypotenuse) right angle triangles with the opposite 5ft in length (1/2 of 10ft). The acute is calculated as tan(60) = 5 ft / a -> a = 5 ft / tan(60) = 2.88 ft. So the Azure Kinect camera needs to be at least 2.88 ft from a 10 ft subject to completely visualize the subject.