Hi @Tracey Coates ,
For now, SharePoint online modern experience doesn't support customizing the column width of left navigation.
But you could use JS codes to customize the width of left navigation in SharePoint online classic experience.
For Reference: How to expand width on quick launch navigation bar
If an Answer is helpful, please click "Accept Answer" and upvote it.
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.