In the context of posters and picture frames, aspect ratio refers to the proportional relationship between the width and height of the image or the frame. The most common aspect ratio for posters and picture frames is 3:2 or 2:3.
If you want the artwork in 3:2 (landscape), you can print it in the following sizes:
IN: 17x11, 24x16, 30x20, 36x24, 40x27
CM: 30x20, 45x30, 60x40, 90x60, 120x80