In some use cases description field isn't very important. However this field is the largest in a new issue form. You can reduce its size.
Just go to the application.css file of the theme you are using and include this code snippet at the end of the file:
Change “50” to your desired number.
Please note that you might need to clear your browser cache to see the change.
You may also include this tweak by using the View customize plugin, creating a view of CSS type and including this snippet. You won't need the “!important” tag: