nods at Melikia :)
oh yeah, it's eyeball time for such things I'm afraid ;)

Scales aren't set by the item size, just by the mat itself's settings.
there's no exact way you can predict how a mat will fit an item, EXCEPT if you "UVMap" and texture it, which is a complex damn thing only done for important models, say, a plane etc.

So it's down to the "Mark 1 eyeball" (tm) ! ;)

world mapping, by the way, is good for mats you want to vary a lot, say, rocks on a river bak, you don't want them ALL to be the same, so use World mapping

you can, by the way, use greyscale images, to drive where materials will exactly be placed on a terrain (scale etc of the mat is another issue completely)
using a black and white image to drive a mixed material

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