Silver standard is great and it values gold more, so you do not need to carry much around.
I did it like this:
If we look at today prices of copper,silver,gold and platinum and make a coin of same size(volume) and start with copper, then
silver is almost exactly 100 copper
gold is 145 silver, this can be minted in smaller coins to get ×100 value
but platinum is only 0,75 gold.
We can say that in middle ages or and d20 realm platinum is worth a lot more, but best solution would be:
1. Silver standard
2. One silver is 100 copper
3. One gold is 100 silver or 10.000 copper coins.
4. Gold coin is about 70% volume of silver/copper
5. If we take 9g for silver(50 coins per lb) as standard we have:
Copper coin 7,7g
Silver coin 9,0g
Gold coin 11,6g(70% volume of other 2 coins)
or if we want same weight coins for calculating bulk, then we go with standard 9g per coin and we get
Silver(standard): 9g, 1× volume, or if we go with 2 euro coin that is 2,2mm thick as standard we get diameter about of 2,2cm
Silver: 9g, 22*2,2mm
Copper: 9g, 24*2,2mm
Gold: 9g, 16*2,2mm