Basically a RTG converts nuclear decays into power by absorbing the nuclear decay products, generating heat. This is quite similar to the process of radiation p

Basically a RTG converts nuclear decays into power by absorbing the nuclear decay products, generating heat. This is quite similar to the process of radiation poisoning, where chemical bonds are destroyed by the decay products. So for a rather handwaving estimate, we can assume that the processes are the same. The lethal dose of radiation is about 3-5 Sv, that is 3 to 5 J/ kg absorbed radiation (times some factors, which describe how the radiation is absorbed and which type of radiation). Therefore assuming full body irradiation and gamma particles, the lethal dose for an adult would be received in a matter of minutes ( 60kg irradiated by 1 W would be 1 Sv/minute). Note however, that a more realistic scenario for a dirty bomb from a RTG involves ingestion of alpha emitting particles for a rather large number of bystanders. ( And therefore the effects vary a lot by the details of the bomb and the radio isotopes used. ) So already with the radioactive material from a rather small RTG you can build a quite potent dirty bomb, rather independent of the exact type of the RTG used.