Firstly, when you book this tour through tripadvisor, you are not provided the full information (like where you meet) that you need, so you have to contact the company. Not the company's fault, tripadvisor needs to fix. Also, note that it is a 48 hour, not 24 hour cancellation notice, if you need to do so.
It clearly states in the description that the "small-group tour" consists of 15 people. We had 18 people and I'm sure three less would not have made a difference because it felt like a large group. When I think of small, I think of less than ten. We were at the back of our group the majority of the time, so who knows what our guide was saying. We could really only hear him when we made a stop.
We booked the afternoon tour, which got started by 2pm by the time everyone filled out paperwork and got fitted for their bikes. Our tour guide was the self-proclaimed "Best tour guide with a stutter." If a severe stutter is going to prevent you from enjoying the tour, then I suggest you make sure to check before you make your reservation. He was very nice and informative when we could hear him. The whole tour lasted about 2.5 hours. I would definitely recommend biking Central Park. It is the quickest and most efficient way to make your way through the Park.