The issue is linked to the language you've set in your android device.
Most probably you set the language as English (United-States) and with these settings you only get the american format in you "Paper Size" dropdown.
Therefore if you want to have the ISO size but still keep your phone in English, you just have to set the language of it in "English (United Kingdom)" and you're problem will be solved.
Manipulation :
Settings > Language & input > Language > English (United Kingdom)
Close your apps and try again, in "Paper Size" you should now see the ISO formats.
Cheers.