Volume/thickness
The web page I usually refer to also shows "concentration," one indicator of how thick the ice is. Lower concentrations tend toward chunks with sea between them.
Also, by noting the areas of multi-year ice from one year to the next, or analyzing the speed of freeze-up or melt, a pretty good idea can be arrived at on the thickness of a particular area.
P.S. There are radar and other systems that can measure ice thickness, but most of the ones I've heard of are aircraft-mounted. I'm not sure if any satellites are carrying such thickness radars.