I suggest a variety. 1 oz coins, 1 1/2 coins. There are even some sheets you can buy that are scored so blocks can be clipped off in small amounts. Sheets are 100g and some are 10 x 10 with 1g sections or 2 x 5 with each section 10g. They're called CombiBars. Another thing that could be useful is silver shot. It's like little beads that can easily weigh out to certain weights. Small coins (1/4 and 1/2oz) are cool to have, but are a little more expensive as compared to spot price. I'm not a big fan of bars.

Gold can be bought in as little as 1g increments, as well as platinum. That's fairly affordable if you're investing.