I haven't made an exhaustive survey, but I checked the major outer-planets missions.
The Galileo spacecraft was designed with a 4.6-meter diameter high-gain folding dish antenna which was supposed to deploy somewhat like a folding umbrella; however, it jammed and didn't deploy fully, and the mission had to rely on a smaller antenna. I believe this is the biggest dish antenna flown to interplanetary space.
The runner-up and thus largest successful interplanetary dish would be Cassini's 4-meter HGA.
Voyager 1 & 2's HGA dishes were 3.7 meters; Pioneers 10-11 2.74 meters; New Horizons 2.1 meters.
Russia/fUSSR and China don't seem to have attempted anything past Mars, and I'm assuming that inner-planet missions don't need such large dishes, but it's possible that I've overlooked something.