This can be done through css - within the /appserver/static directory of the app you want to store the css file in.
for example, if the dashboard is in the search app, then /etc/apps/search/appserver/static/example.css.
Within the dashboard xml
Then your css file will have the changes to the text input that you want.
For width changes-
width: 200px !important;
I encourage using inspect element and and selecting the text input and adding css you want to preview what the changes would look like and to finetune it to what you require. Hope this helps a little.