Ok so I went last year for the first time.. Here is a best estimate for crowds,
MGM Studios holds 7000 parking spaces - during that week they always fill it plus make extra room by using some of the driveing paths.
so if you figure there are a min. of 3 people per car multiply by around 8,000 cars = there is around 24,000 people at least in the MGM park per day. It gets tight!
Waits for fast passes go quick - once they have sold so many - they stop giving them out.
We got to the parks 2 hours after they opened every day and by the time we got there around 10am, fast passes were already up to 9pm at night,
Waits for rides (Non fast pass) were a min. of a 2 hour wait.
Rides like Soarin at Epcot - were 3 hour waits.
Crowds are unbeleivable there - but if you plan it just right - by getting the fast passes first - then looking at the boards to see the wait time for rides.
and doing rides during parades and ALWAYS utilizing the Extra Magic Hour - you can do it.
Also, Remember that at this week - most parks are open till midnight for the holiday and new years. MK is usually 1am and 2am. Use this to your advantage!
BEST ADVICE --> Make your reservations at the 180 day mark. Most of the resturants dont charge a cancellation fee - so figure out what park you will be at on what day and plan your meal. If you change your mind when you are there - at least you had a reservation just in case. (try to plan certain parks around their magic hour in the evening - by midnight - the crowds a way less) Resturant waits with out resv. are also over and hour or two - having a reservation is the best thing you can do for yourself...
OVER PLAN your vacation - Plan which park for what day - plan on breaks, meals and little visits back to your room... It may seem crazy to plan everything.. But with the massive crownds - It will be worth it and will go off with out a hitch!
The best thing we did to make it all work - We rented a car..
Good luck!
