How tall would the average chasmfiend be, and how much would they weigh? On a scale from ant to Godzilla.
So, they're big. Not godzilla big, but larger than elephant big. On average, they're going to loom over you at about twenty feet high, which is deceptive to their size, as they're longer than tall. And some do get even bigger.
Weight, though, is a tricky matter with greatshells on Roshar. The symbiosis with spren is how they get around crushing themselves. (Even on a lower gravity planet like Roshar.)
One last question though, symbiosis is a two way relationship. The chasmfiends get a huge benefit, the ability to not immediately die. So what do the spren get out of it?
Yes, the symbiosis is a two-way relationship. You'll find out more in future books, but suffice it to say, the spren DO get something out of the deal. (At least, when it happens naturally.)