I suppose you mean in Photoshop print with preview.
Paper size is defined in the printer setup. Hit print in PS and the printer window will open, click on paper tab, select roll or sheet. Under paper size select user defined and another window will pop up where you can define the size and save for the future.
Hope this helps as I have a close relative of the 1290 (875) and a 2200 and both dialogues are very similar, so the 1290 shouldn't be too much different.
Rich