EIGRP Packets are carried in IP Packets using protocol 88. Max length of the EIGRP packet is derived from the Max IP MTU on the interface.
Each EIGRP packet carries a 20byte header followed by the body that holds the various TLVs.
TLVs carry multiple pieces of information such as K values, hold timers, control information, and route reachability information. Flags, Sequence number, and Acknowledgement number fields are the ones that provide most RTP functionality in EIGRP.