File system quota for App Service hosted apps is determined by the aggregate of App Service plans created in the region and resource group. We can create one or more standard app service plan which will give you multiples of 50GB (50GB * x) file system storage. We can decide not to deploy anything in the additional app service plan you created, just use it to increase your storage size. Remember that it should be in the same region and resource group.
There is a good extension which would help you to identify the total size consumed in a particular web app. Go to your app service (web app) –> Developer Tools –> Extensions –> Search for Azure Web Apps Disk Usage Site Extension and install the extension.