See this article for general information on FSLogix: What MSPs Need to Know About FSLogix Profile Containers
With a recent update to FSLogix (v2105 https://docs.microsoft.com/en-us/fslogix/whats-new) you can now update existing VHD(x) container size limits by editing the registry on your template/hosts. This change can be made by editing the SizeInMBs value in the FSLogix registry under: HKLM\SOFTWARE\FSLogix\Profiles
This change can be made directly on your templates, hosts or deployed via GPO with a registry edit after updating to FSLogix v2105 or above on your template/hosts.
Please also review our Community for additional details on this topic
For more information on FSLogix container default settings and configurations, see the Microsoft documentation article: Profile Container registry configuration settings - FSLogix | Microsoft Docs