
Hi @Adam Watson ,
AFAIK, there is no OOTB way to set a size limit to a file for external sharing. All external sharing settings we have access to are shown as below via the URL:
https://<tenantname>-admin.sharepoint.com/_layouts/15/online/AdminHome.aspx#/sharing.
As to your requirement, you have to develop a solution by yourself. Or you can share your idea here: SharePoint UserVoice. It is a place for customers provide feedback about Microsoft SharePoint products. What's more, if a feedback is high voted there by other customers, it will be promising that Microsoft Product Team will take it into consideration when designing the next version in the future.
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