Because Microsoft Office applications used physical dimensions like inches and centimetres instead of pixels to size objects, you need to do a bit of math to work out what size to set your icon to before saving it as a picture.
These are the metric and imperial equations for PowerPoint :
size on slide in cm = pixels / 150ppi * 2.54
size on slide in inches = pixels / 150ppi
For example, if you wanted a square shape of 1500px wide by 1500px tall then you would need to size your square in PowerPoint to 10″ x 10″ or 3.94 cm x 3.94 cm.
Caveat : due to rounding errors, you will get very close to the output size but not always exactly as required and an external image editor may be required to crop and resize as required.