1.5mm2 singles work quite nicely out of the box, 2.5 tend to tangle, requiring a second person to keep untangling.

Small packs of wire/cable seem to be a ripoff no matter where you are - here I found 10m 1.5mm2 NYM (roughly equivalent to a hypothetic #16 NM) for €14.50 and 50m for €14.90... guess what I bought!