Is this dialog from InDesign?
Ah man didn't even put the program I'm using huh, it's photoshop cs6.
Great thank you. Have moved this to the Photoshop forum. They will be able to help you.
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A pixel does not have a universally agreed physical size (a pixel's size depends on the particular document's resolution) so there is no reason for "pixels" to be available as an option for document size unit. ("Columns" is available as a width unit because "column" is defined in Preferences as a multiple of a physical unit.)
Your document has pixel dimensions specified at the top of the Image Size dialogue, anyway.