A friend said he's going to Denmark to visit the company that made my boat (Quorning Boats, APS) and he offered to bring back some snow. That reminded me of these images I took a few years ago from about 38,000 feet on my return from Spain. The great circle route took us north and these were taken somewhere between Greenland and Newfoundland. I have no reference for scale, but considering we were over 7 miles high I'm guessing these larger chunks were over a mile across. If you look closely some of them have some height too.

It's very easy to imagine how dangerous it would be in a boat that gets caught up in that, given the pressures that could be generated by the moving masses.