DC cabling size

[digital48]

A couple of other threads have prompted me to ask: what is the correct DC cable size (preferably derived from SMA's Sunny Design) for an installation of Canadian Solar 190s (two strings, one of 14 and the other of seven) and an SMA Suny Boy 4000TL with around 16 metres between the two?

 

[SolarCity]

Not run it through Sunny Design but from my calcs I get 2.5mm.

However, I'd run 4mm cable in.