Probably Argentavis that we know of. There have been ones with wingspans of 30 feet or greater but light known. We are finding more all the time however. If it was a rare breed of bird it is very possible we haven't found a full fossil yet to know of it. As far as size goes, something with a 20-36 foot wingspan is massive in size but not always in weight. We know very little about much of what roamed this planet other than large land and herd creatures. We still don't know much at all of what is in the ocean.
Edit- Appears
Quetzalcoatlus - Wikipedia is the heaviest we know of in the 500lb range. That realization has only come about in the past 20 years as well. Who knows what we will know in 100 more.