Proper frame size

Frame sizes seem to get more confusing all the time. The hybrid bike I
have now has a frame size of 22 inches. I like the size. I was looking
at a bike which I like. There was only one size on the floor, and it
seemed small to me. The sizes are 13,15,17, and 18.5. I found an
aritcle on the net by someone called Shelton, but it was confusing
about the right size. If my current bike is 22 then it seems as if the
bike I am looking at is for midgets or kids, but from the description
that does not seem to be the case. Is there a chart somewhere which
would explain these size differences. I seen some that are in metric,
and others that say small medium and large.

Thanks

Tom