Finish my BASH watermarking script

I have partially written a BASH script to watermark JPG and PNG files using Imagemagick. It works fine only there a couple of improvements I would like to make. Firstly If possible I would like to stretch the text slightly to cover about 80% from corner to corner. Secondly I have only written it to work with one image size and it seems there are a few different image sizes, so it needs to use variables to extract the size from imagemagick, set the point size appropriately and of course set the watermark to the correct size. It can be re-written in python if you prefer