Heather, it sounds to me that what you want is to add an additional pipe size to a drop down selection in your attribute table when editing features. If so, try this:
Open your Catalog and navigate to the Geodatabase you are working with and then to the Feature within that Geodatabase that you wish to add the new diameter to.
Right click on that feature and select 'Properties' at the bottom of the box that opens. This will open another box.
At the top click on "Sub-types'. This will then offer a button towards the bottom of the box that is labeled 'Domains'.
Click 'Domains' and you will be able to navigate to your feature's pipe size in the selections at the top. Click on the attribute that you are working with. You will then be able to see what is coded into the domain for that feature at the bottom of the box and you will be able to edit your pipe sizes there.
To finish, click 'Apply' and 'OK'.
I hope this helps.
Gayle