How many packets does the average MMORPG player send/receive per second and what is the typical size? Does anyone know by any chance?

"Several," and "somewhat".

Most MMOs were scaled to work on a modem, which means no more than 4 kB per second (less than that for original EQ). "Broadband only" can go up to 10 kB per second or more (you're unlikely to want to sustain 150 kB/s for long periods of time to a single player :-). Voice chat can easily add 4-8 kB/s on top of that.

You'd probably want to send between 2 and 10 packets per second, unless you're very action oriented (you could go up to 30 or so). Packets will be sized based on how much bandwidth you want to use, and pack the most important updates for that time into them. "importance" in a typical MMORPG is some function of how long ago a previous update has been sent, whether something major has changed in state, and how far away the object is, so that far away objects get updated more seldom, whereas group members and targeted/targeting mobs get updated more often (as an example).