According to here
http://www.cannondale.com/bikes/05/geo-2.html large size is about 19" (D). Also, If you still have your Rockhopper, measure your reach from the seatpost to the handlebars horizontally. Then subtract 62.5 cm (24.6") (shown as C in frame geometry) from the reach you just measured to find the stem length you need. While ordering the bike, you should be able to custimize the stem length according to the figures you found. This should give you not exactly the same but a close height, and reach to your rockhopper.