You can grow (extend) volumes, but not shrink them. By default, Storage Replica prevents administrators from extending replicated volumes; use the Set-SRGroup -AllowVolumeResize $TRUE option on the source group, prior to resizing. For example:
Use against the source computer: Set-SRGroup -Name YourRG -AllowVolumeResize $TRUE
Grow the volume using whatever technique you prefer
Use against the source computer: Set-SRGroup -Name YourRG -AllowVolumeResize $FALSE