I received a quick reply from Aisla. She wasn't aware that 12x16 was uncommon in other parts of the universe. She also said that 11x14 would be okay but add a note that it's not easily available in your area.
Now, I may have stirred up some competition. Darn.
Thankfully, although digital prints are accepted, the prizes are either film based (main prize is a Mamiya 7II, Ilford tokens) or non-specific (Sekonic L308s and Billingham Hadley bags).
Categories are a bit complicated in that only the "Portfolio" category (4 prints) is eligible for the main prize. Other categories are: People; Places; Still Life & Abstract - all one print each - and Black and White Printer of The Year - one print showing off printing skills...
One strange thing: "Entries must not be in tubes. Entries sent this way will be returned." Storage problems?
Why not email the editor, she is a member of this forum.
Simply point out that 16"x12" is not a size normally available in north America, which she almost certainly doesn't realise. As others have said its a European size, 11"x14" is very rare here in the UK.
It isn't fair to expect you to use papers you don't normally use, or to have to buy far larger paper to cut down.