Good call, Fairylover. I wanted to suggest a mile wide, but did not want to get too carried away. But, I can totally see the river approaching that width as it hits Verbobonc.
I am suspecting that there are no bridges spanning several thousand feet in the Flanaess, are there? If we're talking about this navigable river being this wide, Verbobonc does not enjoy bridges, does it? Maybe ferries are more appropriate.