I re-write my question (and addes some others) in the «How to» section, since I found it more appropriate:
I've been doing a map recently, guided one of those great tutorials found here (thanks for that by the way, they are totally awesome!). I've almost finished it, but I encounter one problem: I don't know the exact size of my world. I mean, I «feel» how big it is, but I don't know how to calculate its proportions. And since it's the map of a world i'm writting a story about, this is the kind of things that must be done, so my story will be more coherent.
I though about a way to solve this problem, but in any ways, I'm short of information. For example: I know that some characters will travel on a river (on a boat, not by swimming, duh!) between two points, in about two or three weeks, but I don't know the speed of a ship, so I can't calculate the distance.
Do you have any ideas to help me?
Did you map the ENTIRE world? Or just a region?
If you mapped the entire world you could calculate distances by comparing it to our planet.
If its just a region then a loose calculation could be made by looking up an
average distance a horse and rider could cover in a overland trip.
(a horse can travel approximately 50 miles a day at a walk with rider, sunrise to sunset on easy terrain)
There are a lot of variables, but I think with reasoning skills something plausible will be come to light.
The difference between stupidity and genius is that genius has its limits.