Measuring 15x20cm, 6x8" 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 6x8", you'll need at least 432 x 576px and ideally 1800 x 2400px.
If you are looking for a frame with a mat for an 6x8" photo, choose the 10x12" (25x30cm) frame. The mat will have an 6x8" (15x20cm) opening.
