At its widest, how wide is the fully-operational Death Star?
How big it was in the Star Wars films, is that what you're asking?
The old expanded universe had it to be about 120 kilometers in diameter (it is pretty much a perfect sphere).
The second one was stated to be 160 kilometers in diameter.
(Though previous numbers where 160 and 900, respectively.)
(EDIT: The models used in the films, given their scale, implied 216 km for the first Death Star, and 900 km for the second.)