Measuring 60x91cm, 24x36" 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 24x36", you'll need at least 1728 x 2592px and ideally 7200 x 10800px.
If you are looking for a frame with a mat for an 24x36" photo, choose the 28x40" (70x101cm) frame. The mat will have an 24x36" (60x91cm) opening.
