The first and most obvious questions are when and where. The answer depends on the time of year and your budget. If you have children in school, you're all confined to school holidays, while some older members of your family may not want to travel in the hottest part of the year.
Budget will play a large part of the question of where, and the last thing you want to do is upset or single out a family member with less money to spend. There are plenty of options when it comes to a large family holiday, so look around for one that offers the best value for everyone.
The answer to both questions, as with most things on a family holiday, lies in compromise/ Offer a wide choice of dates and destinations, and go with the majority decision.