The backend vdisk limit is set dynamically, based on the amount of RAM allocated to the controller. If you increase the 'RAM per Controller' setting through the UI in the Settings > Hypervisors > Edit section, this will get picked up by the storagenode during 'diskhotplug restartController <CONTROLLER>'.
Note that this will cause degraded paths, so always ensure that vdisks are fully redundant and in-sync before starting this operation. VDisks must be repaired in between each controller restart to guarantee continuity of access.