ICMP Echo Request and Reply Size

When we send an ICMP echo request we can tell what packet size we want. The size of the data is the same for the ICMP echo reply, which basically returns the same data. We can see this in Wireshark captures.

500 Bytes

Here's the first ICMP packet:

R1#ping 192.168.12.2 size 500 repeat 1
Type escape sequence to abort.
Sending 1, 500-byte ICMP Echos to 192.168.12.2, timeout is 2 seconds:
!
Success rate is 100 percent (1/1), round-trip min/avg/max = 1/1/1 ms

ICMP Echo Request

Frame 85: 514 bytes on wire (4112 bits), 514 bytes captured (4112 bits) on interface eth1, id 0
Ethernet II, Src: ICANNIANADep_d6:b9:88 (00:00:5e:d6:b9:88), Dst: ICANNIANADep_e9:7a:d2 (00:00:5e:e9:7a:d2)
Internet Protocol Version 4, Src: 192.168.12.1, Dst: 192.168.12.2
Internet Control Message Protocol
    Type: 8 (Echo (ping) request)
    Code: 0
    Checksum: 0xe0ef [correct]
    [Checksum Status: Good]
    Identifier (BE): 2 (0x0002)
    Identifier (LE): 512 (0x0200)
    Sequence Number (BE): 0 (0x0000)
    Sequence Number (LE): 0 (0x0000)
    [Response frame: 86]
    Data (472 bytes)
        Data [truncated]: 00000000000264a8abcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcda
        [Length: 472]

The total length of an ICMP packet is explained here: Ping - specifying size

ICMP Echo Reply

Frame 86: 514 bytes on wire (4112 bits), 514 bytes captured (4112 bits) on interface eth1, id 0
Ethernet II, Src: ICANNIANADep_e9:7a:d2 (00:00:5e:e9:7a:d2), Dst: ICANNIANADep_d6:b9:88 (00:00:5e:d6:b9:88)
Internet Protocol Version 4, Src: 192.168.12.2, Dst: 192.168.12.1
Internet Control Message Protocol
    Type: 0 (Echo (ping) reply)
    Code: 0
    Checksum: 0xe8ef [correct]
    [Checksum Status: Good]
    Identifier (BE): 2 (0x0002)
    Identifier (LE): 512 (0x0200)
    Sequence Number (BE): 0 (0x0000)
    Sequence Number (LE): 0 (0x0000)
    [Request frame: 85]
    [Response time: 0,511 ms]
    Data (472 bytes)
        Data [truncated]: 00000000000264a8abcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcda
        [Length: 472]

Above, you can see the data length is the same (472 bytes) and the contents of the data field is the same.

2000 Bytes

Here's a packet of 2000 bytes:

R1#ping 192.168.12.2 size 2000 repeat 1
Type escape sequence to abort.
Sending 1, 2000-byte ICMP Echos to 192.168.12.2, timeout is 2 seconds:
!
Success rate is 100 percent (1/1), round-trip min/avg/max = 1/1/1 ms

ICMP Echo Request

Frame 1005: 534 bytes on wire (4272 bits), 534 bytes captured (4272 bits) on interface eth1, id 0
Ethernet II, Src: ICANNIANADep_d6:b9:88 (00:00:5e:d6:b9:88), Dst: ICANNIANADep_e9:7a:d2 (00:00:5e:e9:7a:d2)
Internet Protocol Version 4, Src: 192.168.12.1, Dst: 192.168.12.2
Internet Control Message Protocol
    Type: 8 (Echo (ping) request)
    Code: 0
    Checksum: 0x2ffd [correct]
    [Checksum Status: Good]
    Identifier (BE): 3 (0x0003)
    Identifier (LE): 768 (0x0300)
    Sequence Number (BE): 0 (0x0000)
    Sequence Number (LE): 0 (0x0000)
    [Response frame: 1007]
    Data (1972 bytes)
        Data [truncated]: 00000000003cc0d2abcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcda
        [Length: 1972]

ICMP Echo Reply

Frame 1007: 534 bytes on wire (4272 bits), 534 bytes captured (4272 bits) on interface eth1, id 0
Ethernet II, Src: ICANNIANADep_e9:7a:d2 (00:00:5e:e9:7a:d2), Dst: ICANNIANADep_d6:b9:88 (00:00:5e:d6:b9:88)
Internet Protocol Version 4, Src: 192.168.12.2, Dst: 192.168.12.1
Internet Control Message Protocol
    Type: 0 (Echo (ping) reply)
    Code: 0
    Checksum: 0x37fd [correct]
    [Checksum Status: Good]
    Identifier (BE): 3 (0x0003)
    Identifier (LE): 768 (0x0300)
    Sequence Number (BE): 0 (0x0000)
    Sequence Number (LE): 0 (0x0000)
    [Request frame: 1005]
    [Response time: 0,257 ms]
    Data (1972 bytes)
        Data [truncated]: 00000000003cc0d2abcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcda
        [Length: 1972]

The length of the data fields is 1972 bytes and the same for the ICMP echo request and reply.