Get a consult at Mass General. They're a national center for dealing with not only acoustic neuromas, but also tumors in the head of all types. They also have access to a broad array of radiation technology and are an all-around great hospital.
To answer the size question, the typical rule-of-thumb cutoff for radiation treatment is around 3 cm, but it really depends on where the tumor is and its shape. Those variables relate to not wanting to irradiate something very important (like the brainstem, for instance) and covering as much of the tumor in radiation as possible, which is very technical and depends on how the different kinds of light beams they use behave in space.
Talk to both a radiation guy and a cutter -- hopefully they'll agree on the best course of treatment. (I sought out opinions from radiation people and they said unequivocally NOT to get radiation, which was very helpful in my decision making process.)
