People have flown light aircraft of nearly every conceivable size across the Atlantic. I even believe some guy made a crossing in an ultralight a while back. I believe that, using the northern routes, the longest required over-water leg can be as short as 600 nm. This makes it theoretically possible for many light single and twin engine aircraft to do it. There are other considerations though - they won't let just anyone make a crossing. You have to meet certain standards, carry appropriate equipment and have special training.