Yes, our 21x30 cm (8 ¼ x 11 ¾ in) frames and posters correspond to A4 size.
30x40 cm (11 ¾ x 15 ¾ in) is close to, but not exactly A3 size. A3 size is 2 cm wider.
A 31 cm poster hanger may be an option.
No, we do not sell A2 or A5-sized products.