I only have the Fairy Mini (the second-biggest). I think it was the right choice for me... the original Fairy just seemed enormous, and the Fairy Mini is a very easy-to-handle size for me.
I wouldn't personally go smaller... if you read Darling Jen's review of the Fairy Miniature (I think?) she explains how they're optimized for "size not performance" according to the packaging, so if you use them for very long sessions they can overheat. The Fairy Mini doesn't have that problem and I assume the Fairy Original doesn't either. Along with that, I've heard the littler ones are more buzzy and less rumbly.