Looking at the only map with a scale in the main rulebook - it is about 230 miles from Altdorf to Nuln. The River Reik looks huge - definitely over a mile wide. Given the fact that fantasy maps are designed for style and aesthetics and not exactly to scale, how wide would you say the River Reik is? Are there any references to it in previous WHFRP products? The only other fact that I can can think of is that there is only one bridge over it at Nuln and the bridge is one of the "Wonders of the Empire."

It is a big river, that is for sure. Size though, is hard to say, a mile be right maybe just a little bit smaller.

I remember reading a warhammer novel or a tabletop army book, forget which. there was mention of massive barges and ships that could fit dozens of people that were passing each other, on the river. also considering that some of the ships from the ocean come down it for trade, it must be a massive river, in depth and width.

It is large river, but width probably changes very much. And every map gives different ideas about this width.

Good example is the very end - Nuln. It is one of the difficult ones. Few notes in the official sources state that Reik could be even LEAGUE width next to Nuln, but from the official Nuln map (FoN) you can easily see, that Great Bridge is about 250-300 Yards (which still makes the bridge HUGE). So, if its league width next to Nuln it becomes very fast VERY thin. Another similar opening could be the Reiksport next to Altdorf...

BUT, this could be so in other parts also. As Facepalm mentioned Reiks average width is probably about mile, but opens sometimes even more and gets smaller in some places.

On Gitzman's Gallery there is a download that has charts for travel distances in the empire by various means. It is gleaned from several sources. They list the average as 310 miles by river.

EDIT: Woops, totally misread the question. I always want to compare things like this to modern equivaents, but that is hard with a river because population density in most areas means high water demands which reduces the size. You could look at the Amazon, Nile, or Congo rivers, but their sizes are extremely dependent on monsoon rains making the seasonal changes in size much greater than I would think for the Reik.