Measuring 48x75cm, 20x30" 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 20x30", you'll need at least 1440 x 2160px and ideally 6000 x 9000px.
If you are looking for a frame with a mat for an 20x30" photo, choose the 24x34" (60x86cm) frame. The mat will have an 20x30" (48x75cm) opening.