You need to calculate a little bit. For example, I work with a screen resolution of 120dpi. To get 1280px x 720px on export to png, I need an object size of
width = 1280/120 * 2.54cm = 27.09cm
heigth = 720/120 * 2.54cm = 15.24cm
If you do not change anything in the export to png, then LibreOffice uses the DPI setting of your system. Because of rounding in the conversion between cm and inch the result might be one pixel off. The export dialog shows 27.10cm width, but the amount of pixel is 1280px here.
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