This is a hard one. The metric system is the easiest for me to relate to but it is also less fantasyish to me (since it is my native system). I can't really decide and forfeits my vote until further notice.
With no conclusion in the poll, let's review and see if some agreement can be made between only using comparative language and if/how to include the metric system.
1. If we include the metric system into the game world, NPCs and players would have knowledge of and be able to use metrics; the inclusion could be covered by vague lore about the level of sophistication of the sciences -- maybe alchemy or Renaissance level engineering.
2. If we include metrics ONLY for out-of-character descriptions such as rooms', ("The white stone wall extends a dozen meters into the sky,") and items', ("The meter long iron blade is connected to a brass and leather bound hilt,") would this be the best of both worlds, or would it be too much of a challenge for builders?
3. If we continue with only using comparative language, will it be able to describe everything adequately? Sometimes it seems to be a little far-fetched or messy for giving a simplistic idea of the scale and scope of things.
If everyone interested in this issue would consider and voice a position, we could surely have this riddle solved. Hehe.
[I'm alright with either three ways, but leaning towards number two; it would require some effort to make sure comparative language remains the method for in-character talking such as NPCs talking about things, but it doesn't seem like such a stretch for a mind to handle? Does any of this make sense? Please ask if these points aren't clear enough.]