Vastitas' Dust Devil - Sol 104

The vertical post near the left edge of this image is the "Mast" of the Meteorological Station on Phoenix.
The DD visible at the horizon just to the right of the mast is estimated to be 600 to 700 meters (about 2000 to 2300 feet) from Phoenix, and 4 to 5 meters (10 to 13 feet) in diameter.

It is much smaller than DDs that have been observed by NASA's Mars Exploration Rover Spirit much closer to the Equator and it is closer in size to Dust Devils seen from orbit in the Phoenix Landing Region, though still smaller than those.
The image has been enhanced to make the Dust Devil easier to see.