When doing electrolysis of NaCl (aq), is the resulting amount of Cl2 lethal, or is the amount miniscule

It depends entirely on how big your equipment is.

If you're just doing a lab experiment where you've got a beaker of salt water and a battery, there's nothing to worry about. (I've done that often.)

If you're an industrialist planning to electrolyze tons of salt water at a time with your own high voltage line direct from the hydroelectric dam, you've got environmental concerns big time.