How difficult will it be to get approved for a Breast Reduction from Regence Blue Shield of Idaho?

Based on your description you may be an excellent candidate for breast reduction surgery.  
The best way to obtain insurance coverage for breast reduction surgery involves some “hoops” to jump through. The more documentation you have (for example, from your primary care doctor, physical therapist, chiropractor etc.) the better when it comes to obtaining insurance “authorization” for the procedure.
This documentation and letter/pictures from your plastic surgeon will help you obtain authorization. Make sure you are  consulting with a well-trained/experienced board-certified plastic surgeons.
 When dealing with health insurance companies, be prepared to be persistent. Keep in mind, that breast reduction surgery tends to be one of the most patient pleasing operations performed.
 Best wishes.

Each BC/BS contract can be different in the same state and city, so the only way to find out would be to get a consultation with an experienced plastic surgeon. After an exam the surgeon will be able to estimate the weight that might be removed from each breast. That will then be communicated from the surgeon's office to BC/BS and the company will decide if they will approve you or not. Cup size and symptoms carry little weight actually. The policy stipulates the weight that must be removed from the breasts BASED ON the patient's height and weight. You are pretty petite so the likelihood they will approve you is good IF the breast weight is enough.