This chamber is actually two knocked into one and so it is very big, possibly around 200 feet across. It is flooded and presumably goes all the way down to D floor, making it about 240 foot deep at the extreme. As can be seen there used to be a bridge, long since fallen, with only a few supports left hanging.
This chamber must be crossed to reach Rhosydd, and the only way to do so now is to throw a dinghy down into the water over the 25 foot vertical drop then attempt to abseil into it (see the little adrift dinghy floating on the water for scale). After this, the water must be crossed to the 25 foot vertical climb waiting at the other end to gain the continuing passage.
(Photo of Croesor Slate Quarry uploaded by Penitent Miner. Please do not copy without permission).