Get Packet Length

I was able to retrieve my packet data as an unsigned char* with

gCharArray = tvb_get_string(tvb, 0, length);

but I have no idea how to determine what 'length' SHOULD be. My message type does not include any size information unfortunately. I want to grab the data from position 0 to the end of the data. How can I accomplish this?

Thank you for your time,
Brandon