Large ones sometimes hold some or all of the following: graduation ceremonies, conventions, used curriculum sales, field trips, holiday ceremonies, workshops for parents and for children. Some of the larger ones are for profit, while the smaller ones only collect for expenses. The larger ones are registered as a business, while the smaller ones are not. usually considered a business.
Small ones are the easiest and least expensive to start, and are usually limited to 8 families or less. They usually meet once a week in member homes with parents taking turns teaching group lessons. If one parent is good in science, she does the science projects
More on starting small co-ops
Brainstorm with other home school friends to see what things members might be interested in: Parents taking turns giving lessons (very popular), mom's night out (usually monthly), play days, field trips.....Have some written by-laws that parents sign upon joining: size limit of the group, will new members need to be approved by the group before acceptance, all children must be accompanied by an adult (that way you don't become an unpaid babysitter), How will expenses be paid for (in advance or after the activity...in advance usually works best), ext
You might consider registering with your state home school association so others can find you. However if you want to limit the size of your group, and you already have enough members, you may decide not to do this.
We are part of one at our church with about 15-20 families and a part of one in the next big town (an hour away) with about 70-75 families. The big one only gets together a couple times a month for field trips (the zoo, maple festival, etc) or for big get togethers (we do an art/music fair, a big picnic, an apple cidering.)
My largest co-op has over 400 families. It is very large. I run one myself. It is small and mostly taught by the parents. We use a room at the library. I teach most of the classes. Parents do volunteer occasionally. The biggest part of running your own co-op is time involved. Each co-op is run differently. Every summer we have a meeting decide about changes we wish to make and classes we wish to teach. Feel free to ask me anything. I am currently teaching a 13 week geography through art class. My sons and I love our co-ops.