A .png file is really the wrong format for this. .png files are raster images, consisting of pixels. Unless the file contains dpi info, there's no real size information in a .png. And even if there were, the resolution of the image file can affect the accuracy. Note that's it's impossible to draw a round hole in a raster image, which is made of of square pixels.
No, if your file is an image of a drawing with dimensions, then that's different.