PS 半張A4既SIZE

半張A4既SIZE=A5 paper
A5 paper is an International / European paper size established by the ISO, the International Standards Organization. It measures 148 x 210 millimeters, equivalent to 5.83 x 8.27 inches.

The number of pixel is not decided by the software "Adobe photoshop" but depends on quality of the image (picture). Using high quality camera or printer, you can have higher quality image as the intensity of each pixel is variable; in color systems.