I have a Patriot Fuel+ that I've used for about 2 years now and it works great.
The iPhone6 has an 1810maH (milliamp hour) battery. This means that you'll want a battery pack that has at least 1810mah to get 1 full recharge of your cell phone (basically double the time). Most of your cheaper battery packs will be around 1440mah which was the standard cellphone battery before this current generation of phones).
If you get a battery pack of 4000maH you'd get a little more than 2 full charges out of your phone (triple the life), 9000maH would give you more than 4 charges.
Another thing to look for is the charging rates. The Fuel+ I have has 2 USB ports, a 1.0a port and a 2.5a port. The 2.5a port can deliver more energy (2.5 times more) than the 1.0a port so your phone will charge faster than the 1.0a port. (This is great if you don't want to leave the charger connected to your phone the whole time or if you want to give your buddy a quick charge while you're having lunch.)
One other issue to remember is the time it takes to recharge the battery pack itself - The bigger your battery the longer it takes to fully charge it (my 7800mah battery takes about 8 hours to fully charge from empty)
Now - How fast your phone drains at DisneyWorld depends on a lot of things - how many photos/videos you're taking, how often you check
MDE or how often you're uploading your photos to facebook (Look, it's me and Gurgi!... Yeah, like THAT ever happens...

). The other issues you run into are the phone trying to maintain its connection with the cell network. With all of the crowds carrying cellphones they're all trying to get onto the same cell towers and this requires the phone to boost its transmitter to try to get a clear connection to the tower. This will cause your battery to drain faster than it normally would.
In my personal experience though a 6000maH battery size seems to be perfect for me. I have a 7800maH battery and I rarely discharge it more than halfway - and that includes helping friends charge up too.