I had a similar issue and didn't find a solution. It was using my faster GPU with less VRAM where I couldn't achieve 512x512. I installed a different tool and edited a file that allowed me to point it at my slower GPU with more VRAM and now I can get 512X704.
Installed this.
https://github.com/lstein/stable-diffusion/tree/78aba5b770d6e85e44c730da9735118d...
Edit the dream.py file and change the last bottom bit of code to point at the GPU you desire.
if __name__ == "__main__":
os.environ["CUDA_VISIBLE_DEVICES"] = "1"
main()