I am based in the US, however, as some of my buyers are in countries that use metric, here are US print size options with the closest equivalent metric size:
11″ x 17″ or A3 (297 x 420mm)
18″ x 24″ or A2 (420 x 594 mm)
24″ x 36″ or A1 (584 x 841mm)
36″ x 48″ or A0 (841 x 1189mm)
My prints are printed with a 3/4″ border, which adds to the sizes noted above. For example, an 18″ x 24″ print will have a total size of 19-1/2″ x 25-1/2″. I typically number and sign prints on the white border. If desired, I can sign on the print itself if requested.
If you need a custom size please let me know and I will do my best to accommodate your request.