Ha! Snuck a peek at one of my partner's forestry field books: coil bound, DUKSBAK brand, made in Vancouver BC,no less!
She says there's several sizes, this one was a little smaller than A5. Sure as Hello not paper. Really nice to write on with pencil.
BTW
North America doesn't use the A-series paper sizes series, probably because they make far too much sense.
We have letter (approx A4) and legal (approx A2?), double letter and a few business card sizes.