GLTF doesn’t really allow custom shaders, you’re mostly limited to PBR style. So for realistic skin you’ll need a diffuse texture and a roughness texture, and will skip the metallic texture and the emissive texture.
Since you’re running on web I highly recommend using 2k resolution at most for your textures (always keep textures at a power of two, so 2048x2048 px). Even better if you can use 1k textures. It all depends on how closely the user will be viewing the model. Compress the textures as jpeg instead of png for optimal transmission size.
I’ve spent more than half a decade on work exactly like this so definitely feel free to ask me to clarify if any of this was confusing.