Measuring 48x60cm, 20x24" 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 20x24", you'll need at least 1440 x 1728px and ideally 6000 x 7200px.
If you are looking for a frame with a mat for an 20x24" photo, choose the 24x28" (60x70cm) frame. The mat will have an 20x24" (48x60cm) opening.