You can start by checking to see if they have a Convention and Visitor's Bureau or Chamber of Commerce in the area in which you plan to have your family reunion. Ask what type of services they offer to family reunion groups. They should be able to help you locate venues like hotels, banquet halls, etc.

Also, send out a letter and survey to your family members to see how many people are interested in attending this event. This will give you an idea of how many people to plan for.