Will I Loose HA with a vm that has an RDM?
No, HA works with RDMs.
To add HA will work as long as all hosts in the HA cluster can access the all LUNs that is used by the VM including the RDM LUNand because hosts see the LUN DRS and vMotion will also work
This is only if the RDM is Virtual and not physical right? Which limits the RDM to 2TB. I need a 5TB RDM.
both vRDMs and pRDMs work with HA and vMotion.