Hello mousewalker,
Welcome to the HP Forums! I understand your concerns. I am going to provide the Photosmart C5580 Product Specifications. This document does list the supported custom size paper.
What is your operating system on your computer? Windows or Mac?
If you have a Mac computer, then you have more options to set custom size paper. I will provide a HP document on performing this step here.
If you have a Windows computer, then you will have limited options to set custom size paper. I will provide a generic HP document on Changing the Default Paper Size, Type, and Source Settings.
If you have any further questions, please let me know.