How do I resize a Longhorn Volume

Resizing a Longhorn volume is as simple as modifying the PersistentVolumeClaim size which causes Longhorn to automatically expand the volume as long as the PVC’s storage class has allowVolumeExpansion set to true.

NOTE: You must scale down anything mounting the volume for the volume expansion operation to succeeed

See - Longhorn | Documentation for more details on Longhorn Volume Expansion