A particular implementation of an IEEE 802.3 Physical Layer is denoted using a three field notation. The three fields are data rate in megabit/second, medium type, and maximum segment length in hundreds of meters. One combination of of 802.3 parameters is 10BASE5 which specifies a 10 megabit/second transmission rate, baseband medium, and 500 meter segments. This correspondes to the specifications of the familiar "Ethernet" network.
The MAC header contains 6 (2) octets of source address, 6 (2) octets of destination address, and 2 octets of length. The MAC trailer contains 4 octets of Frame Check Sequence (FCS), for a total of 18 (10) octets.
IEEE 802.3 networks have a minimum packet size that depends on the transmission rate. For type 10BASE5 802.3 networks the minimum packet size is 64 octets.
IEEE 802.3 networks have a maximum packet size which depends on the transmission rate. For type 10BASE5 802.3 networks the maximum packet size is 1518 octets including all octets between the destination address and the FCS inclusive.
This allows 1518 - 18 (MAC header+trailer) - 8 (LLC+SNAP header) = 1492 for the IP datagram (including the IP header). Note that 1492 is not equal to 1500 which is the MTU for Ethernet networks.