Harry Potter and The Puzzle of the Potions

In the first "Harry Potter" book, Harry and Hermione are trapped in a room with a table that has 7 different bottles on it. They have to find the bottle to let them move forward through the room and the bottle that lets them go back.

[7 bottles on a table]

On the table is a roll of parchment that reads:

            Danger lies before you, while safety lies behind,

            Two of us will help you, whichever you would find.

            One among us seven will let you move ahead,

            Another will transport the drinker back instead.

            Two among our number hold only nettle wine,

            Three of us are killers, waiting hidden in line.

            Choose, unless you wish to stay here forevermore,

            To help you in your choice, we give you these clues four:

            First, however slyly the poison tries to hide

            You will always find some on nettle wine's left side;

            Second, different are those who stand at either end,

            But if you would move forward, nether is your friend;

            Third, as you see clearly, all are different size,

            Neither dwarf nor giant holds death in their insides;

            Fourth, the second left and second on the right

            Are twins once you taste them, though different at first sight.

Hermione remarks: "This isn't magic -- it's logic." and soon works out which is the one that takes a person foraward and which will take a person back. Can you?