Measuring 45x60cm, 18x24" 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 18x24", you'll need at least 1296 x 1728px and ideally 5400 x 7200px.
If you are looking for a frame with a mat for an 18x24" photo, choose the 22x28" (55x70cm) frame. The mat will have an 18x24" (45x60cm) opening.
