The biggest problem I encountered was the applying of the mirror modifier.
Some vertices in the middle went wrong. And a lot of faces in the UV-map turned to green (stretching problems). But I fixed them, by merging vertices. And adding more seems to the model body and legs. After this no problems at all.
Also, a strange effect was that all my object where uniform scale 1. But they did UV unwrap that way. I needed to select all objects, and apply the scale.
After seeing this image, I think we need a texture map of the size of 4k. Because the face has about a space of 800px square. Too low I think. But will see.