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
