How can i set topic image' width

You will need to inspect the CSS on the browser and amend it, applying your customisations to a Theme or Theme Component.

This is the realm of Theme development.

Take a look at Beginner's guide to using Discourse Themes to help you with the Discourse specifics.

However, this is largely a general web development technique which is not the focus of this forum.

You might want to take a look at some guides like CSS Tutorial or CSS first steps - Learn web development | MDN