Azure Static Web Apps offer several plans, including a free plan and a standard plan (and a dedicated plan which is currently in preview). Each plan comes with specific quotas and limits, one of which is storage capacity.
infoYou can find more details about these quotas here: Quotas in Azure Static Web Apps

If you’re wondering how to check the storage usage of your Azure Static Web App, I recently discovered a way while exploring the Azure Portal.
Checking Storage Usage
Instead of estimating storage usage from your local build output, you can check it directly in the Azure Portal:
- Go to the Azure Portal.
- Search for your Azure Static Web App.
- Click on Diagnose and solve problems.
- Select Configuration and Management.
This page provides various details about your Static Web App, including a Storage Usage section that displays the current storage consumption.

I hope this helps you better understand and monitor the storage usage of your Azure Static Web App.