Hi, you can use the show command using the output modifiers and get the number of packets and the number of bytes. I am not a math wiz but with some division you should be able to calculate the packet size. Give the following command a try. Hope it helps

int#show int fa0/22 | inc packets

5 minute inputrate 1000 bits/sec,2packets/sec

5 minute output rate2000bits/sec,3packets/sec

18622 packets input, 1687332 bytes

0 input packets with dribble condition detected

25468 packets output, 2193290 bytes, 0 underruns

int#show int fa0/22 | inc bytes

MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,

19316 packets input, 1748213 bytes

26342 packets output, 2260220 bytes, 0 underruns