Changing width property of a :before css selector using JQuery

I have a page that has an image in it and I styled it using :before CSS selectors.
The image is dynamic so it hasn't a fixed width; So I need to set :before rule's width dynamically.
I want do it in client side using JQuery.
Assume this:

How Can I only change the width property of class with selector (and not one without it) using JQuery?


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