I live in Canada, and when you mentioned A2 sizing as fairly common, I had to Google that because when I see pre-made frames for sale at art-supply shops, camera stores, dimensions are in inches and are along lines of 8x10, 11x14, 20x24 etc. I cannot recall seeing any A-sized frames here on the shelf so to speak. The top Google hits all seemed to be for UK based stores so I take from that A2 type frames are going to be common in Europe.
I don't see any problem sizing your work to fit into common (standard) sized frames and if A-sized frames are common in your locale then go for it. One advantage to this approach would be ability to sell photographs with mat only and let buyer get their own frame that matches their decor. Added advantage is this approach can also keep your costs down (frames, particularly if custom sized are expensive) and therefore your selling price down which can be difference between sale and no-sale.