Hi Monk55. A 5 x 8 mm cisternal-component AN is small. Take a look at your MRI and get a feel for its growth rate. You can the MRI on disk which makes measurement easy. If your symptoms are mild, there would be a lot of room before brainstem displacement and even more before brainstem compression.
In Denmark and England, 95% of tumors less than 15 mm in cisternal diameter are not treated. To get to this figure, you've got 7 mm up your sleeve. You've had your tumor for 5 years, so chances of rapid growth (>5mm) are small according to the literature.