Hi and welcome Archana -
combining AN surgery with BAHA surgery pretty much depends on the doc involved. My neurotologist doesn't typically combine the surgeries - so I had retrosigmoid in 5/07 to remove my AN (about the size of yours) and BAHA surgery 9 months later in 3/08. I would have had the BAHA implant earlier, but spent lots of months battling my insurance company for coverage. Thankfully they paid for everything 100% in the end.
Although I originally was hesitant to have another surgery after my retrosigmoid, the BAHA surgery is absolutely nothing like AN surgery. They don't open up your head - and it's a short outpatient surgery. I even had mine done under local anesthesia.
Bottomline, if your doc combines the surgeries - great. But if he doesn't, don't let that prevent you from getting the BAHA.
I absolutely love my BAHA; IMO it's amazing!
Best,
Jan