Logging payload length and part of message

Given that you want to see client request and server response sizes, you could do something like this:

when CLIENT_ACCEPTED {
    TCP::collect
}
when CLIENT_DATA {
    set cs_len [TCP::payload length]
    set cs_payl [TCP::payload]
    TCP::release
}
when SERVER_CONNECTED {
    TCP::collect
}
when SERVER_DATA {
    set ss_len [TCP::payload length]
    set ss_payl [TCP::payload]

    log local0. "Client side ($cs_len) = $cs_payl, Server side ($ss_len) = $ss_payl"

    TCP::release
}

This is logging the entire request and response payloads, so you'd need to customize as required.