Hi @Anonymous,
If you want to call the Graph API to change the storage quota of the SharePoint site, I am afraid that there is no way to achieve this in Power Automate currently.
As an alternative solution, I think you could run the PowerShell to create a site and set the storage quota using Azure Automation and a runbook.
If you set up that piece you can then create a Flow with the action Azure--> Create Job to run PowerShell. You can then allow your users or admins to run that Flow from within SharePoint, say from a site request list.