i took a regular sized die and filled in the holes with wood filler. after painting 3 sides red and 3 sides blue, i weathered it with a light sandpapering on the edges.
Well, I think the size of a casino die would be ok. It's bigger than a regular d6 (Nerd alert!), but considering you only use one at a time the size shouldn't be a problem.
I like Beesa-Pesal's idea. Easy enough for anybody to make one.
I'm guessing around 19mm (.75") I may be completely wrong on this, but I made a chance cube back in February using these measurements and it looks pretty size-accurate compared to when Watto is holding the cube in his hand.
Here's a screengrab of Watto holding the chance cube:
From this image, the cube looks pretty small, hence my estimate of 19mm (.75"). This is also one of standard sizes for a die, so I'm hoping that this was the size of the original.