I don't think there has ever been a map that zooms in far enough to judge the width. Its certainly smaller than Zundbridge a little further along by the Rat Hills. I would say it is not a major bridge. Probably only wide enough for 2 or 3 wagons. If I were going to make a guess I'd say its a moderately deep river (it is fed by its own headwaters as well as the South Ardeep) and runs into a large river delta. This means it needs to carry enough sediment that it doesn't just all wash out into the sea. I'd guess 60' to 150' + across. It would also most likely not be a fast flowing river.
Thank you for the answer! I just read the Storm King's Thunder campaign. There's a small village south of Ardeep Forest, named Nighstone. The village is build on it and uses the river as a moat. There's a map of this village and the river is 40/45' large.