You can generally get more detailed information from the command line: -
toaster*> stats explain counters fcp fcp_write_size_histo
Counters for object name: fcp
Description: Histogram of FCP write sizes
Size: 10 column array
Column names: 0-511, 512-1023, 1K-2047, 2K-4095, 4K-8191, 8K-16383, 16K-32767, 32K-65535, 64K-131071, > 131071
That specific counter is a diag counter and you'll need to be in diag mode to view the details.
The properties say it is a delta so it'll be the number of writes of each size since the last sample period, which looks to be every minute.