You will probably be ok, as marins have huge head tubes in comparison to other bikes...it why ive always had them... if its a 19" marin sizing it will be a 150mm headtube....so realistically you will just need a longer seatpost! Most bikes that size will be a min 20mm smaller on the headtube, which really makes a difference if your tall.
At 6'4" i only got off a 19" last year as my daily ride!
Hope its ok. They are a great ride...