How could I get the total size of a some data that has been transfered using UDP over a wireless channel from a Wireshark capture trace ? I have read somewhere that the sender advertise the total size of data to transmit before sending any actual data. However I could not spot that in my trace.
UDP is connectionless, i.e. the data consists of distinct UDP datagrams. There's no advertising in UDP itself but it's possible that the application level protocol on top of UDP uses something like that.
If you filter the Wireshark capture for the source IP address and UDP destination port you should get a good idea of the total data transmitted.