I will assume that you are already familiar with the RDM disks if you are not you can have a look at this link. About RDM

In-order to increase the RDM disk size, first you need to increase the LUN size from your storage manager console. Once it’s increased you can confirm the increased size from the VCenter console by selecting the

ESXi host>Configuration>Storage Adapter

Then select the storage adapter either iSCSI/FC

Here I have selected the iSCSI adatpter and it is showing the LUN size expanded by 6 GB. Once this is confirmed go to the server that has RDM disk mapping, Then go to the Disk Management utility and perform a disk rescan this will display the additional disk size available for expansion.