Istria ( Croatian , Slovene : Istra , Italian : Istria ) , formerly Histria ( Latin ) , is the biggest peninsula in the Adriatic Sea . Istria ( Croatian , Slovene : Istra ; Italian : Istria ; Istriot : EÃ stria ; Hungarian : Isztria ) , formerly Histria ( Latin ) , is the largest peninsula in the Adriatic Sea .