Measuring 30x60cm, 12x24" 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 12x24", you'll need at least 864 x 1728px and ideally 3600 x 7200px.
If you are looking for a frame with a mat for an 12x24" photo, choose the 16x28" (40x70cm) frame. The mat will have an 12x24" (30x60cm) opening.