Hi saniab,
I have not given up, I did some additional research and found some interesting information.
Media Types Supported Paper (brochure, inkjet, plain), photo paper, envelopes, labels, cards (greeting)
Media Weight Supported A4: 60 to 90 g/m²; HP envelopes: 75 to 90 g/m²; HP cards: up to 200 g/m²; HP
10 x 15 cm photo paper: up to 300 g/m²
Media Sizes Supported A4; B5; A6; DL envelope
HP Deskjet Ink Advantage 1015 Printer Specifications Table
It seems to me that the specification page on the Deskjet 1015 support page is inaccurate. A5 is not a supported media size. I am sorry to be the bearer of bad news but I am glad we finally understand why you are not getting the option.
I hope this resolves your inquiry.