Atlas Mapping with the whole model selected makes only sense for a shadow map.
For a character, you should choose pieces of the model, f.i. the face including the ears, the chin, the forehead and throat. Then you should get a reasonable mapping of it. Same with a hand or a leg choose one half of it.
This way your skin is separated in different bitmaps.
You can reduce the amount of needed place by erasing one half of the model, layout its UVs and afterwards copy, paste and mirror the other half to re-complete the model.
An absolutely different way is to export it to blender, where you select and save seams to get proper chunks of the mesh for mapping.