There are lots of water sources along the trail when I did it. However there are areas when you are going over passes where there won’t be any water sources for a few miles.
I always carry a minimum of 2L of capacity during any hike, but that’s me. For the highline trip, I carried a 3L camelback style bladder since I stay hydrated better when I can drink more often from a tube. I didn't always fill it up to capacity, only filling it up with 2L most of the time. I suppose if you were cautious about filling up before any of the major passes, you'll be good with just one liter. However, empty 70 ounce platy bottles weigh an ounce, so why wouldn't you stay on the safe side with a little extra capacity if needed.