Will this be covered under my Magna care insurance?

Your beast bet is to check in with your insurance company and find out what criteria they use.  Cup sizes are not used to plan breast reduction, rather you need precise measurements made by the surgeon and an estimate of the amount of tissue to be removed.

You need to check with your insurance company for their criteria to approve the surgery, then go see a plastic surgeon.

This should be a covered service unless your union has pulled all breast reductions out of its obligations. MagnaCare is a third party administrator and many of the plans predate ERISA so they can include these limitations. See a board certified plastic surgeon who can start the preauthorization process and bring along all your supporting documentation.

I am sorry to hear about the problems your disproportionately large breasts have caused.  Breast reduction tends to be one of the most patient pleasing operations we perform. Best to check directly with your insurance company to see whether breast reduction surgery is a “covered” benefit with your specific policy. Then, given that every insurance company has different “criteria”, best to identify exactly what your specific insurance company considers criteria for approval. Generally, 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. 
In regards to outcome of the planned procedure, the most important decision that you will make will be careful selection of your plastic surgeon. I would suggest starting with the American Society of Plastic Surgery and/or the Aesthetic Society of Plastic Surgery to obtain a list of well experienced board-certified plastic surgeons.
Then, I would suggest you visit a few surgeons whose practices concentrate on aesthetic surgery. Ask to SEE lots of examples of their work AND preferably speak/see patients who have had similar procedures done.
Be very careful with your decision-making. You will find, while doing your due diligence, that there are many different “specialties” who will offer their services to you; again, I strongly recommend you concentrate on surgeons certified by the American Board of Plastic Surgery.    
 Before undergoing the breast reduction procedure it will be very important to communicate your size goals with your surgeon. Most patients wish to achieve enough of a reduction to help with their symptoms while remaining proportionate with the remainder of their torso. With the goal of improving communication with my patients I find the use of photographs of “goal” pictures (and breasts that are too big or too small) very helpful. I have found that the use of words such as “natural” or “B or C cup” means different things to different people and therefore prove unhelpful. Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup size may also be inaccurate. 
Best wishes.