No problem.

There are three ways (in my opinion) to look at the river width issue. One is to make it as wide as it actually is for the scale of the map. In that case, it would be only the finest of lines, no matter how wide it is. Even the mighty Mississippi, which drains everything between the Rockies and the Appalachians is only a couple of pixels wide at this scale in a satellite view. Other rivers are only visible at all because the land around them is somewhat greener. That's not really practical for a map, though, and this isn't a satellite style, anyway, so I think you'd be safe to ignore the physically accurate approach.

The second is to make it as wide as the people who interact with it perceive it to be. The people see it as the biggest river they can imagine, so on their maps, it's drawn quite wide, much like you have it here.

The third is to link the feature's size to its importance. There are two ways you can go here: how important is it to the cartographer, and how important is it to the story?

All told, I don't think it's necessarily a bad thing for the river to be oversized in comparison to the map's scale, but if it is, it should be for a reason. Even if you make up the reason afterward! ;-)

Oh, and if this is going to be printed in a book, remember that it's going to be rather small. So don't go overboard with details. Take a look at some maps in existing fantasy novels, especially softcovers to see what looks good to you and what looks overwrought. Tad Williams' Memory, Sorrow & Thorn trilogy is a good example: I really like the coastlines, mountains and hills, but the forest seems muddy to me.