The only way to read that programmatically would be if the vendor exposed this information as an attribute via their XML file. I looked at a couple of Basler aces that I have, and none of them seemed to have that information. They have an attribute called PixelSize, but that refers to bits per pixel and not the physical size. I skimmed the GenICam Standard Features Naming Convention to see if there is a standard GenICam feature for what you're looking for and didn't see one. Unfortunately it seems like you won't be able to get that information programmatically.
thank you for your quick response and investigation.
Instead I'll read the camera model name and define the pixel size for all possible cameras.