This Add-on makes it possible to precisely control the width & height of images in Google docs. It currently only works with ‘In line’ images. I’m waiting for feedback by the Google API team to make it also work with ‘Wrap text’ and ‘Break text’. (have been waiting forever so don't think this is going to happen.)

Use the menu Add-ons->SetImageSize->show sidebar to activate. Then select an image in your document and select "get size" in the sidebar. Now change the size to your liking, then select 'Apply' to make the change.

If you get the error "ScriptError: Authorization is required to perform that action." then maybe you are logged into your browser with multiple accounts and Google script doesn't like that.
Aug 17th 2019
- added link to privacy policy
July 18th 2018
- clarified that only in-line bitmap images are supported (eg. Google drawings are not).
June 28th 2018 
- added function to retrieve original image size [1:1]. Should work with bmp, gif, jpeg and png.
- stopped decimal places in presets.
June 19th 2018 
- added an additional authorization to allow the sidebar to open. This was a breaking change that Google made and which stopped the add-on to work from June 7th.