Ethernet frame format is
Preamble |
sfd |
destination address |
source address |
length |
data |
CRC |
Here destination address and source address are 6Bytes each.
Length is 2Bytes and 32 bit(4byte) CRC is used.
Preamble+ sfd( start frame decimetre)=(7+1)Bytes
If we include preamble and sfd then Ethernet frame size becomes 26 bytes.