well you are only going to be able to change the cog up or down 1 tooth without changing the chain.

I would put on your smaller cog, and then (w/ the chain broken) hook it around your ring and your cog with the wheel pushed all the way back in your dropouts. Then fasten the chain there. Don't use a half link. Remember, don't push the pin all the way out when breaking the chain.

Then, when you put on a bigger cog, the wheel SHOULD be able to sit in the front of your dropouts.

I usually just take an unhooked chain and wrap it around the drivetrain to determine the proper length.