As big as you can find. The biggest problem is with the overall length of the cartridge. To run the bigger weights means usually having to either use it as a single shot or upgrade to a detachable box mag setup that allows for longer overall cartridge lengths.
I have used the 200gr Accubond loaded to OAL 3.6 for about 5 years with no complaints. I have a friend who shoots the 210 bergers but seats them out long and just feeds one at a time. Some people seem to get them to shoot well and still fit in the factory box. Congrats on the new RUM, it is an impressive round regardless of what size bullet they are throwing.