Here goes the (not so) silly question! Sizes and dimensions!

[Lukc]

I also usually take the side of art and topology, rather than necessarily topography.

For features ... average sizes and so forth ... I tend to rely on experience and places I've visited. If I imagine I need a river in the desert I might be reminded of the Nile, I'll go on Google Earth and measure it's width and a given location and so forth, to get an idea of the size. If I want mountains, I've been to different types and sizes of mountain, I go check out some typical mountains and measure them again. But ... this is more relevant for city maps, where I find some scale is more important than on a more symbolic, large scale map.

[Morgan]

So for now the general feeling seems to go in the artistic direction rathen that geographical accuracy.
I must say I agree, especially for fantasy maps. Perhaps one should consider the purpose of a map before deciding how much accuracy he wants to achieve.
As Jaxilon pointed out, most scaled rivers would not show up on large regional maps, but a fantasy novelist or a RPG narrator need that information, even if it's not accurate. In this context, less accuracy means more freedom to change things to fit the dramatic turns of the story, and that's ok. That's my vision, I'm glad to find so many others that think the same way.

Lukc, actually your advice is valid also for regional maps. I mean, I asked for average values so that in the process of building a semi-realistic world, I know how big can be certain features in the map, but perhaps the best way to do this is not to face the whole map at once. If I have a Sahara-like zone, I can delimit borders, and then use Google Maps to measure features in real Sahara. In this way I don't even need to ask myself where a certain feature can be. For example, if I see the Amazon river is the widest on Earth, how would I know if such a river can also exist in another climatic zone? Thanks for pointing out!