Dear, Michael!
I was facing with following assumptions:
typical SIP INVITE – about 1375 bytes (without UDP (or TCP) and IP overhead) and about 1400 bytes including UDP (or TCP) and IP overhead.
typical SIP 200 OK – about 800 bytes including UDP (or TCP) and IP overhead.
typical SIP 100 Trying and SIP ACK – about 300 bytes including UDP (or TCP) and IP overhead.
Examples of call scenarios 1 and 2 you may find out in IMS 3GPP or TISPAN specifications.