From 2008 International Tornado Class Rule
GENNAKER MAX DIMENSIONS
Luff length - 9150 mm
Leech length - 8050 mm
Foot length - 4250 mm
Head to Mid-foot - 8750 mm
Half width – as defined by ERS G.7.5(b) - 3450 mm

Then apparently the max girth allowed is 81% of the foot lenght, but actually there is no minimum