I think the Bulkley is the largest tributary, then next is likely the Babine. The Suskwa appears smaller than the Kispiox. I've never seen the Sustut, but at average fall flows the lodge guides run jet sleds on it.
Of course there is the matter of how you define the size of a river. Is it by mean annual flow? Drainage area in square miles, kilometers, or hectares, etc.? Or how wide it is compared to your longest cast in the places you fish it. See what I mean?