Device used to measure stellar radii I remember seeing in a video an instrument that was used to measure a star's radius with nothing more than the light it emitted. 
The device worked by opening or closing its mechanism in increments, while watching a display. The display started out showing some shape, maybe some lines or a circle, with blurriness. The closest you could get to making the shape blurry-less would determine the instrument's approximation of the star's radius. 
My question is, what is the name of this instrument?
I am not sure what you are referring to; possibly it is intensity interferometry using the Hanbury Brown &amp; Twiss method, but even this requires two telescopes. This kind of interferometry was used to estimate angular diameters for some tens of stars but is not generally used these days for this purpose. Instead, one constructs interferometers from separated telescopes using delay lines to compensate for path-length differences and maintain coherence between the different paths that the light takes. This then uses amplitude interferometry to reconstruct the angular diameter of a star from the interference pattern seen in different telescope baseline configurations.

The radii (or to be more exact, the angular size) of many nearby stars and also of many giant stars at rather larger distances, have been measured using interferometric techniques. An accurate distance is then required to estimate an absolute radius.

Relevant papers are by Boyajian 2010 and Boyajian et al. (2013). There are a number of such interferometric experiments, mostly working in the infrared.

There are also direct radius measurements for stars that are occulted by the Moon and other solar system bodies - the time-dependent diffraction pattern can yield an angular size for the object.

Absolute radii are also provided by eclipsing binary systems (independent of whether they have a known distance).

See also How do astronomer measures the size of any celestial objects?