One of the best sources on European boat specs is the Texel handicap rating measurement form.

http://www.watersportverbond.nl/data/837_numdet_27-5-2007.pdf

This is the PDF version but you can also download the excel version and see the actually measurement taken. It is a very rich source of information and a headache for builders. Afterall the REAL specs are publicized after being measured putting truth to claims.

For the Inter-20 sailing in the EU we find.

nacra inter 20 - 18,00 sq mtr. with 9,08 mtr luff
nacra 20 medium - 18,56 sq mtr. with 9,00 mtr luff
nacra 20 large - 19,18 sq mtr. with 9,00 mtr luff

It think the nacra 20 medium was a try out and the nacra 30 large is the current "popular" US rig that is being used in the EU now. OF course the "nacra inter-20" was the old size and formula 20 compliant.

15 sq ft = 1.39 sq. mtr.

and I'm convinced it is better for sailing light at 320 lbs (145 kg) in anything but sub 8 knot winds.

Wouter