Measuring 40x48cm, 16x20" 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 16x20", you'll need at least 1152 x 1440px and ideally 4800 x 6000px.
If you are looking for a frame with a mat for an 16x20" photo, choose the 20x24" (48x60cm) frame. The mat will have an 16x20" (40x48cm) opening.
