UPS output max cable length
The size of the wire you need depends on the load connected to the extension cable, not the source providing power to it.

The ampacity of the wire will not change based on the voltage of your electrical system, but the power handling is directly tied to it.  If you are expecting to get "nearly" 1200w over a 6M cable at 230V, you need a wire that is rated to handle 5.2A.

14AWG (2.5mm^2) copper wire has an ampacity of 15A if allowed to undergo a 60°C temperature rise, and would cause a loss of around 4.22V at 5.2A. (this in turn means a loss of nearly 2% of the total power available being dropped in the wire)

TLDR:

A 2 or 2.5mm^2 extension cord is all you "technically" need

The larger the wire you use, the smaller the losses (up to a point) so a 3+ sqmm wire would be better. 



keep in mind that an extension cable is not a replacement for an outlet, depending on code in the area you live you may be in violation of any insurance you may hold on the property if you use an extension cord for "permanent installation".

I'd definitely recommend contacting an electrician rather then using an extension cable as a full time power cable.