Measuring 20x30cm, 8x12" 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 8x12", you'll need at least 576 x 864px and ideally 2400 x 3600px.
If you are looking for a frame with a mat for an 8x12" photo, choose the 12x16" (30x40cm) frame. The mat will have an 8x12" (20x30cm) opening.