it's an outdoor stadium that hold 10,000. depending on the time of year, you'll want there to be there an hour early. the shows fill up, and they do turn people away. if they have 2 shows that night, the second show has a lighter crowd usually.
It is an outdoor ampitheater. On one of our visits the show was cancelled one night due to "inclement weather" (we're not just talking rain, it was good old fashioned thunderstorm with lots of wind and lightening). It is always a good idea to arrive an hour or so prior to show time to make sure you get a seat. They do allow standing room only people after the benches are full, but eventually they start turning folks away. I've seen the show max out even on a slow day. It's really popular for a reason, however. It's a great show!
We saw Fantasmic on our last trip in January. The time of year you are visiting will make a HUGE difference in your arrival time. When we were there, we arrived about 30 minutes eary, had a great seat, and there were seats left in the theater when the show began.