Variable data lengths in monitor

Hello,
I have a driver and monitor that are parameterized to handle different sequence item types. In one type, the transfer item can send out read or write commands over the bus. This is determined by which start of frame character is used. The two commands have different lengths, only the write has an end of frame character, and the read is expecting a response. In another transaction type, there is just 6 bytes that get sent out with only a start of frame character. My monitor is watching the transmit lines and the receive lines(they are separate). How can I tell my monitor what my expected data size is?