I think the brain stem location may complication things as well! I totally agree, I would get several opinions.
"Average" growth for a meningioma is 1-2 mm per year. In general, they are considered to be slow-growing tumors. Following that, and assuming it started to grow immediately after it was discovered, then it shouldn't be any more than 4 mm. However, often, they do not behave as they should. It seems that they sometimes go through growth spurts, sometimes associated with hormone events like pregnancy, breastfeeding, or menopause. Sometimes they grow rapidly for no apparent reason. Some meningiomas have a high growth rate because they are not typical. A small percentage of meningiomas are atypical (Grade II) or even smaller percentage are malignant (Grade III). These may grow faster than "normal" as well.
I personally would be leery of just watching a meningioma pushing on the brain stem, unless it is considered inoperable. Many factors are considered when looking at treatment. One of them is age. If the patient is old, and it is unlikely that the tumor would grow enough to have significant impacts during the patient's lifetime, then watching is an option. Radiation or gamma knife may also be a good option for an older patient.
If I were you, I would get a second and even a third opinion. Seek out neurosurgeons who specifically deal with meningiomas and who specialize in tumors in that area.