statistics volume show has a field for op size. There is also one for statistics lun show. However you can always take the bytes/second divided by # of IOPs.
For example, 21,626,880 read byte/second divided by 330 read IOPs would be 65535.
Note: stats volume is limited to 64k because the WAFL file layer does 64k. At the protocol level it gets split up into multiple 64k chunks. You can use stats nfsv3/smb2 or stats lun to get a readout of op size > 64k.