Hello JaimeB,
Welcome to the HP Forums! I understand you are trying to set up a custom size on your Officejet 6700. What is your operating system on your computer? Windows or Mac?
I am going to provide the product specifications here. If you click on "Paper Handling Specifications", then click on "Supported Custom sizes".
You can only perform a custom size on a Mac computer, not a Windows computer. If you have a Mac computer, then please follow this HP document on Printing to custom or special paper.
If you have any further questions, please let me know.