What do they do if a wound is too deep or too wide to stitch?

Heals from inside ou. Wound is left open to heal from inside out - sometimes with a help of wound vac.
Depends on the wound. A deep wound can be sutured in several layers. Wound that is too wide is sometimes left to heal on its own or plastics surgeons will use a flap. An infected wound is usually left open to heal by itself.
Wound Closure. If you have a wound that you believe is too deep or wide, you should be seen by a professional either in an emergency dept or urgent care setting. I cannot think of a wound too deep. If it is a deep wound, it can be sutured in layers. As for too wide, wounds to wide can be approximated and closed by secondary intention or may even require a skin graft.
Age? Cause? A wound may be too old to stitch also. In wound care we encourage a wound to fill itself by carefully controling how moist it is and protecting it with many possible dressings possibly including suction. A simple but deep one may need deep and superficial sutures while a surgeon may essentially change the alignment of parts of a wider area of tissue to enable closure . Called flapping or bring.
Laser treatment. In one of my cme course at harvard medical school, 'structural acupuncture for physician' they have mentioned that laser beam at certain wave length helps wound healing. I have tried on not so deep, but more than skin thickness, involving fascia did heal quicker. But I have not attempted on larger and deeper wounds.