I have gone through a real pain to figure out how to print an exact size in Mac OS X. Follow the steps below and maybe, just maybe, it'll work for you. I used the following steps to print the correct size in 8.5 x 11 paper
Open the image in Preview, it might help if you image size is actually 8.5 x 11.
Go to Print...
Select to Paper Size and Manage Custom Sizes...
Create a new size with width 8.5 and height 11 inches. Make sure all the margins are set to 0.
Click OK and select that template (a couple of times my settings would change automatically to something else, so you better keep checking that the same setting is selected)
In Preview Option,
Unselect Center Image
Scale -> Fill Entire Paper
In Paper Handling Option,
Select Scale to fit paper size, and also make sure that Destination Paper Size is the new template that you just created.
Unselect Scale down only.
That's it!! It worked for me, hopefully it'll work for you too...
