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