I get the same kind of result as the example here when I use a VAE made for SD 1. Try setting SD VAE to none. As for your question, https://platform.stability.ai/docs/features/api-parameters says:
- 1024 x 1024
- 1152 x 896
- 896 x 1152
- 1216 x 832
- 832 x 1216
- 1344 x 768
- 768 x 1344
- 1536 x 640
- 640 x 1536
You can try other multiples of 64.