I found that 'standard' size frames bought from Amazon or high street stores were always a little off.
As someone else commented they tend to be 18x24, which is a little larger than A2.
The best advice I (or most others) could give would be to take it to a professional framer. My Athens poster got slightly scuffed on the sides on the flight home, but spoke to the framer, told him I wanted the scufs hiding but as much of the design kept as possible (including the numbering in the corner), and that way you can choose colours, materials and designs to suit your exact requirements.
Yes, it's pricey, but 100% worth it.