A 1/2 real is a bit smaller than a dime. I don't know if Spain ever made a 1/4 real which would be about the size of watch batteries I'm used to 
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Spanish: 1/2 Real; Madrid mint; 1.69 grams (originally) and 15 mm in diameter.
Assayers: JF - Juan José García Caballero y Fernando Vázquez
Don........