See pages 53 & 54 of manual.
If custom size unavailable, then suspect printer does not have that capability.
I would guess you can insert custom envelopes by adjusting edge guides to accommodate.
Select nearest envelope size. Printed address would appear higher or lower than normal position.
Best I can do for you.
Here is your manual:
http://www.hp.com/ctg/Manual/c00591689.pdf