According to Daophot2.pdf

Datapars and Daopars contain 3 size-like variables

fwhmpsf - full-width at half-maximum of a stellar object 
	  (PSF - point spread function)
psfrad  - "radius" of the PSF

fitrad  - How much of the PSF used in making the fit to the star.
	  Stetson recommends fitrad ~ FWHM