Jane
Welcome to our club, "the one no one want to join," as is said often here. Many people will respond to your post I'm sure. "Small" can be anywhere from a few millimeters to 1.5 or so centimeters ... I think that's an accurate description. 1.5cm is about the size of the tip of your little finger. Mine is 1.5cm by 1.6cm, considered on the large side of "Small."
In addition to being small, almost all ANs (acoustic neuroma = vestibular schwannoma = vestibular neuroma) are very slow growing, like 1 or 2 mm a year. (As with everything, there are a few people who've had faster growing ones). This means you probably have lots of time to do the research you want before deciding how to proceed, AND it probably allows you to do the half marathon. The caveat, of course, is if your symptoms start getting worse more quickly and affect your ability to do a half marathon. (Now, that wouldn't be a problem in my case, because there's NO WAY I'd EVER be able to run a half marathon, with or without an AN !!! ;-)
There are three basic options you have (with lots of subcategories you'll learn about): 1) wait and watch 2) physical microsurgery or 3) radiation surgery (usually a one-shot outpatient procedure, but there are kinds that involve several outpatient radiation appointments).
And, finally, you've found a GREAT SUPPORT community, one which will provide lots of information sources and conversations, as well as lots of emotional support. I recommend you join the ANA organization (I can't remember if you have to join in order to participate on the Forum, in which case you already belong!). They'll send you any or all of the informative pamphlets they have that you want.
So, take a deep breath, and gather information about your own AN - size, location, what nerves it might be pressing against, etc. One thing I didn't know until this happened to me was that you can get the pictures from your MRI on a CD; mine included the software that allows you to open up the picture files .... allows it on a PC computer, that is. (I was told it would work on a Mac also, but mine doesn't.) There's lots to read online, including posts here. Main thing is, you're not alone while dealing with this, and there are many both compassionate AND clear thinking folks here. I have found it to be a godsend.
Take care.
Dana