We started out with a pack of gold, silver and bronze Fimo, we rolled it out like you might with cookie dough.
Then cut out small circles for the Knuts, medium ones for the Sickles and big circles with the Galleons with circular cookie cutters (the cookie cutters are in the back of the below photo).
Pressed the name of each coin and some decoration with some magical themed charms. (see the letter stamps and charms in the below photo)
Fired them in the oven and let them cool.
The on the bronze and silver ones I did a black wash to give them that 'ye olde' look. And on the gold one I did a wash of a darker shade of gold. (the black didn't look very good)
This should give you a sense of the size of the coins. And yes, I know some of the real Harry Potter coins are not round, but I only had access to round cookie cutters, but if you don't have round ones then don't dispair, use whatever you want!!! Have fun.