UltraRes lets you render at higher resolutions than SDXL would normally allow. It supports txt2img as well as upscaling images in img2img to bigger image sizes than previously possible. Without the lora distortions occur at these resolutions.
Supported txt2img Sizes:
1536x2048, 1344x2016, 1280x1920, 1248x1824, 1536x1536 (and their landscape versions)
Recommended Lora Strength:
0.5 to 1.0: The higher the resolution, the higher the lora strength has to be to avoid distortions. Higher lora strength means less sharp details, so keep it as low as possible and as high as necessary.