I think it's about right. Maybe 2 metres more for each 7-10kg. It might change a bit with wind conditions too. Maybe if it's blowing 30 knots things are different than if it is blowing 18 knots.
I personally think that for about every 10kg in body weight, you need another 2 square metres of kite. eg In 17 knots, a 70kg guy can be powered nicely on a 12m while an 80kg guy needs a 14m kite.
I agree. A company which sets up an online forum which doesn't work properly is a company which obviously doesn't have its act together. It takes a lot of techy expertise to design and build a kite. If the website and forum and crap, it kind of indicates that the kites are crap.