This Add-on makes it possible to control the pixel width & height of images in Google docs.
Use the menu Add-ons->SetImageSize->Set image height and Add-ons->SetImageSize->Set image height. You will find a number of menu entries to set the height and width of an image in pixels. The aspect ration of the image will be maintained.
Use Add-ons->SetImageSize->sidebar to open a panel on the right. With ‘Get’ you retrieve the width and height of a single selected image. You can then change either width or height (aspect ratio is maintained). With ‘Apply’ the width and height are applied to the image. In the sidebar you will also find shortcut buttons to set width or height for multiple images.
Width and Height are set in pixels. I have not found a way to set in other units (mm, cm, inch, etc).