We could lose him for the 2019 Asian Cup which runs from Jan 5th to Feb 1st 2019 as Australia have qualified for the finals. I don't think there is any tournament January 2018 but could be wrong.
Ryan is 6ft 1/2inch.
David Ospina, Tom Heaton, Julian Spreroni, Steve Mandanda, Kasper Schmeichel, Claudio Bravo, Willy Caballero and Michel Vorm are all between 6ft and 6ft 1 so while Ryan isn't the tallest keeper out there i don't think it's much to be concerned about.