is still worth fighting over, if you and your brother both want it.
What's the smallest piece of lego?
A eurobricks forum suggests it is a flower petal.
Lugnut says a gold coin, weighing in at just over 0.056 g; the flower petal is a hefty 0.064 g.
For reference, a 1x1 plate is 0.176 g and a 1x8 brick is 3.06 g.
You'll need a lot of legos to build a lego mosaic. I don't even want to thing about the cost of lego bricks per kilogram, or maybe I will; this package of 25 1x8 bricks has an MSRP of $6, or about $80/kg, or $36/lb. This discussion on classic-space suggests mixed lego goes for $9-12/lb in 2009 dollars.
LEGO is a registered trademark owned by the LEGO Group. And don't you forget it.