Here is an example for you:
1.Click Edit page in the Settings gear.
2.Add a web part, choose Script Editor web part.
3.Edit web part, then click EDIT SNIPPET, add following codes. Note: You should replace the column display name with your own.
<style>
.ms-vh-div[DisplayName='custom']
{
width : 700px !important;
}
</style>
4.Result:
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.