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I suspect that, whatever software you are using, you need to set a 'Custom Size' for the book. Standard sizes may not include the 12 by 12 you want, but Custom Size may allow you to set 12 and 12 and the dimensions.
What software / program are you using?
Where is the size of 11.00 by 8.50 inches listed? in Adobe Photoshop, in its Print settings? I would presume so.
If so, try going to
Control Panel - Devices and Printers
and right-click on your printer's logo there, then choose Properties.
In the printer's Properties (otherwise known as the driver settings), look for Paper Size, then choose Custom, and specify 12 by 12 inches. See if, going back to Photoshop, that makes the 12 by 12 size available again.