Multiple users have requested some sort of tool to help identify wide row keys. They get into a situation where they know a wide row/partition has been inserted and it's causing problems for them but they have no idea what the row key is in order to remove it.
Maintaining the widest row key currently encountered and displaying it in cfstats would be one possible approach.
Another would be an offline tool (possibly an enhancement to sstablekeys) to show the number of columns/bytes per key in each sstable. If a tool to aggregate the information at a CF-level could be provided that would be a bonus, but it shouldn't be too hard to write a script wrapper to aggregate them if not.