Its either 19" or 17.5".....and the head tube is too small for a 19".
They measured from centre to centre of top tube NOT centre to top of seat tube. It may measure 18", but as a marin catalogue size its going to be smaller. If you measured to the top it could be even less!
The only reason i say that is the geometry is different from size to size. This is in the catalogue and may be important as marins have loooong top tubes.
All tyh best with the sale, they are great riding bikes imho.