I go to WDW with extended family about once a year (though not every year) withgroups from nine to about 13. upwards of thirteen, it becomes a challenge just to move from place to place. Someone has to use the restroom, someone want s a snack...
Also, don't know how to say this, but I have learned the hard way large family getherings are SO much easier if you avoid the major holidays. If you go near Xmas, try to book tehtrip to avoid the worst of the holiday crowds as much as you can (go a few days early?) Disney/airlines/car rental/etc totally reem you pricewise.
I've been really hammering this lately on the DIS, but waht made our most recent trips really fun was staying club level. If you are already doing deluxe, look into it. Club level gave us a good space where we could meet up. It also gave us extra service - which helps. We've done plenty of sucessful non club visits, but it was a nice perk. It also takes a chunk of burden off me.
I think the other important thing to communicate up front is the idea of compromise. Everyone is going to have to compromise. We do largely stick together. We don't do strict rope drop - but we try to be at the parks fairly early most days- we also try to do some late nights (esp MK and Epcot). I don't plan ride order, except to start days with an e-ride and fp another e-ride.
We eat one TS a day. This is not negotiable. Therefore, I often like the dining plan. Once we are there, everyone knows the deal. However, the
DDP and club level is too much food. You know your group best.
We also make a point of basically going on all the rides at least once. I hate it when part of the group refuses to go on kiddie rides while the kiddies have waited for the big kids to go on the thrill rides. We don't stand in lines for characters, unless they are very short. We do one character meal per trip. DO use a fp 'runner.' As soon as we are inside the gate- one person collects all the park tickets and leaves to go get fp for the group.
One thing, getting tables for your large group can be an extra wait. If you can dine early you'll be better off- esp during a busy time like Xmas. Avoid lunch noon to one.
No promises on rooms being together. However, it really helps the overall vacation if everyone is in the same resort and room category. I think it is one time where going deluxe is a nice addition. Deluxe transportation is easy- esp if you pick a BWK area deluxe or a monorail resort. If you can't go that way....then consider the new Animation resort or Pop - preferred location rooms. The values are streamlined and efficient. That is good with big groups.
Another thing that really helped our group - having a minivan. If you can get your group in one vehicle, it helps so much. Maybe two mini vans if your group is too big for one. Much of the time, we use Disney transportation - but having a vehicle for things like dinners is nice. Even if half the group takes the bus and half the group can take the van- it can really help the old and very young keep with the group.
Also, as group leader, I find I have to be group mediator sometimes. It takes a level of restraint. I also have to keep in mind it's not personal. It just is.
I like it when we can travel the parks together. If we split up early, then try to get together in the parks to tour, it gets to be a hassle. If you are going to tour toether at all - start off the morning together. That way the fp's are in sync. It also helps if you TRY your best to keep everyone peeing at the same time. (when one person goes, everyone tries to go). Do split ups in the afternoon, before dinner and/or after dinner. Cell phones are a big help. It also helps to give every adult a copy of the schedule that they keep on their person. (one page- wallet sized)
It also can be lots of fun to do one special event. Grand gatherings just lists all the reservations together, and allows YOU to make changes if needed. It doesn't have to be more than that. Your special event could be a photography session. That's a really nice thing to do if the whole family is present. Another could be something like a group boat ride, a special park tour...