Measuring 30x48cm, 12x20" is a common size for photo prints, art work and posters. The standard way to measure a print is by measuring width by height.
To get the best print result for 12x20", you'll need at least 864 x 1440px and ideally 3600 x 6000px.
If you are looking for a frame with a mat for an 12x20" photo, choose the 16x24" (40x60cm) frame. The mat will have an 12x20" (30x48cm) opening.