turnip26
The clue is in the name, resolution. If you use 256, or 320 but your footage is 4K then your face when merged will be blurry because the model has to be upscaled in pixels to match the resolution of the source image. For best results, you need a sharp video where the face in the video is a close match to the resolution of the model, so unless you have modern hardware which supports high-resolution models stick to low res source footage. I would say anything at 256 or less should only be used with 720p.
The other tip is to start with a pre-trained model using settings which match your footage and on a new project with the same settings start with the same model. Even if the faces in the new project are completely different you’ll be amazed at how quickly the model will update to the new faces and give a decent result.