Looks like incision is opening. What can I do to help keep it from getting larger? (Photo)

It appears that you have a small wound separation.  Call your PS to find out whether this should be left open or reclosed.  Many factors go into this decision including how long that it has been open, what is beneath the incision (like an implant) and what caused it to open (like ugly drainage).  Your PS needs to decide.  

Go see your surgeon as soon as you can. If your surgeon has given you formal instructions for wound care then be sure to follow those all the way. You can also call your surgeon if this is a weekend and request instructions for wound care and supplements/vitamins which might help.Good luck, Dr. ALDO 

I would not be too concerned as it is only a small part of the wound and not uncommon following a breast reduction. I would recommend protecting the wound with steristrips and a occlusive dressing and refrain from any physical activity. This would ensure the wound stabilises and is able to heal.

its always best to see your surgeon about it since he/she knows what was done and is bestt managed by your surgeon.  As for preventing it from getting larger, this is something beyond our control but if its a minor issue, it will not keep expanding.