Yep, how small you can go depends on how many charges you want to get out of it. I bought a small/lightweight charger from Staples for my recent trip to
Disneyland Paris because I wanted something compact, and it works fine (it's Staples' own brand). It only charges 1 to 1.5 times, though, so it worked fine as long as I was faithful about charging it every time I was back at my room, but I got frustrated by still having to mind my screen time to avoid draining the battery more often than I could recharge it, and when I let a friend use it to charge her phone I had to be very frugal with my phone's battery that day. I usually prefer to travel with my
KMASHI, which I refer to as my "power brick"--it's as long as my iPhone, nearly as wide, and much thicker. It weighs as much as a brick of that size would. It also holds enough charge, though, that I was able to keep my phone charged for the entire duration of a 17 hour park day without making any effort whatsoever to conserve battery.
You also have the option of buying a FuelRod onsite. It's a lightweight charger, not huge capacity, and once you buy it you can swap it out for a fresh one whenever it runs dry (there are kiosks around property). Hopefully someone who's tried it can chime in and let us know whether they thought it was a good value (it sounds like you pay $30 once and swap it out as many times as you want indefinitely...but maybe there's a hidden charge I don't know about?).
Edit: The KMASHI link I put in was supposed to direct to
Amazon, but it goes to
Walmart...? It's out of stock on Walmart's site but not on Amazon. At least that lets you get the full name and see what it looks like, I guess.