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Frank Sinatra might not have flown to the moon, but he could bounce up there on his personal pogo stick ... because Paul Anka says he was very well-endowed.

The singer-songwriter confirmed the rumors about the crooner's impressive member during a conversation with Page Six ... telling the outlet it was huge before adding, "I don’t know what that does for you!" Getty

Anka says he would hang out with Frank and the rest of the Rat Pack in Las Vegas regularly, and they'd all get in the sauna together ... during which he joked he "had trouble with eye contact."

Sinatra's ex-wife Ava Gardner allegedly once said Sinatra may have been just a skinny, 110 pounds, but "10 pounds of [his weight] is c***" ... a quote which has lived on for years, even if it might not be total

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