Measuring 20x25cm, 8x10" 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 8x10", you'll need at least 576 x 720px and ideally 2400 x 3000px.
If you are looking for a frame with a mat for an 8x10" photo, choose the 12x14" (30x35cm) frame. The mat will have an 8x10" (20x25cm) opening.