Any insights on what should be the transistor size supporting the 1T1R ?

I'm not even sure what is the preferred size for the ReRAM itself. The layout in the PDK repository follows the minimum size guidelines, but I haven't seen any information to indicate what happens if you make the device larger, connecting to more vias, for example. Assuming a single via above and below the ReRAM device, you have 9kohms through the vias on top and bottom alone, added to the resistance of the ReRAM device itself. From that, you should be able to determine the minimum size transistor needed to deliver the amount of current required to program the device.

Exactly. I have been trying to figure out the current requirement since the figures supplied speak about the voltage that should be suppled to the WL/BL/SL terminals of the transistor but not the current. Now , since the W/L will change the current and the voltage requirement as well , a calculated decision needs to be taken.