How large should the corpus be to optimally retrain the GPT-2 model?

I just started working with the GPT-2 models and want to retrain one on a pretty narrow topic, so I have problems finding training material.

How large should the corpus be to optimally retrain the GPT-2 model? And what is the bare minimum size? Should it simply be as large as possible or can it flip over and make the model worse in some way?

I am also not certain how many steps you should let the retraining run. I have been using 6000 steps when testing, and it seems not much happens after that, loss only moved from 0.2 to 0.18 last 1000 steps.