In the initial design phase for Phoenix, I made the decision to use the metric system for play. This is partly because I have some international players, and partly because I think the customary system is kind of annoying in day-to-day activities. Why should anyone have to remember numbers like 1760, 5280, 16, 12, and 3 when they can just remember 10, 100, 1000?
Still, I have years of training against me. It's a lot more natural to say, "You enter a 10 foot by 10 foot room," rather than, "You enter a 3 meter by 3 meter room." And while it is easy to make a meter the same as a yard, it is a bit harder for some players to convert kilograms to pounds in their head.
For those of you that have played internationally for more years than I, did you play in feet or in meters?
For the most part, it isn't really relevant in Phoenix anyways, so I can easily get away with using most any form of measurement for descriptions. A hex is 1m, which can just as easily be 1 yard.
Phoenix abstracts weights and avoids issues of tracking excel spreadsheets of items. This abstraction is similar to the Rule of Assumption. Most items are not specifically counted up, and characters do not have a maximum carrying capacity. It is left to the GM to determine if someone is violating common sense.
"No really, my gnome wizard is carrying around that 12-foot golden statue we saw 4 rooms back."
He better be using some sort of dimensional magic!
Armors have weight classes, which are simple integers in the 1-5 range, and a character's Strength attribute relates to the heaviest wearable class.
In the current working draft, characters can carry a number of coins equal to their Strength score times 100. So, if a character has a Strength of 10, they can carry 1000 coins - which might be silver, gold, copper, or any combination thereof. I'll see how this works in play once characters more regularly hit the limit, but I will drop this rule if it adds unnecessary complexity.
These design decisions are all based around the sort of gameplay I want to inspire in the players - Phoenix is about heroic interactions and epic combat. Whether a sword weighs 3 pounds or 5 pounds or 2 kilograms is not relevant to that sort of gameplay.