Realistically you'd be able to train up to 760M param models. For that you'd need 32gb+ VRAM GPUs which I think AWS might have. You can try iwth 16gb VRAM GPUs, but you would need to figure out FP16.

https://github.com/shawwn is doing some work in the GPT-2 space including using TPUs instead -- which has given him pretty good results.