Two of my potted citrons have each produced a single large fruit this year. Both plants are about 4' (1.4m) tall; both fruits about 4" (10cms) in diameter.
Rhobs el Arsa is a variety originally found in Morocco, but my plant probably originates from the citrus collection in Menton, France. It is shaped like a flattened sphere, but often has a secondary mound of growth at the bottom.
I propagated the second plant from a tiny piece of fruit stalk given to me on the Greek island of Naxos in 2004. See
http://www.homecitrusgrowers.co.uk/citrusplaces/naxos.html
I now believe this is probably the citron also grown in Crete, where its Greek name is translated as 'Cretan Smooth' (Λεία Κρήτη).
If you're a greek scholar you could look at
http://nefeli.lib.teicrete.gr/browse/steg/theka/2011/GalaniMarietta/attached-document-1305108147-485865-16210/Galani2011.pdf
Mike/Citrange