jack wrote:

The barbeque sized ones; about how much are they full and empty?



Look for the letters "TW" (tare weight) stamped near the top of the tank.
Right after that is stamped the empty weight in pounds.
The barbecue sized tanks are nominally 20 pound capacity tanks,
which means that you can put in slightly less that 20 pounds of
propane.

Typical numbers might be (YMMV): 19 pounds empty, 38 pounds full.