it depends on the backing store (which is pluggable). I would not want to exceed let's say 50Mb in a Y.js doc, but i've tested the in memory storage with graphs approaching 1Gb and it's been fine - like any graph it really depends on how you query it. Most of the docs I'm dealing with in production are less than 10Mb, so this is fine for my use cases, but... buyer beware!