Yes, Ozbungy made a good summary
And he is right - when you LEARN to ride a directional, you need the same kite as on your TT, for starters.
But only untill you've learned, then you will typically go one size down for the sweet spot.
The upper end is not affected much really, but as soon as you get the hang of it, you wont ride deliberately in the upper end no more, ever, on a waveboard - as this will kill every chance for riding waves freely
By the way, a 7m2 is definitely not a "small" kite at all, and for 70kg on a waveboard when you've learned the ropes, it is actually at least a "medium" kite size.
4-5-6m2 is smaller kites, in waveriding terms.
As said many times, the most used kite size in waves is around 7-8m2 if you have changing wind conditions, for average weight 80-85kg riders.
Can be used in almost any conditions
A 9 or 10m2 is considered big in waves, and most lighter than 75-80kg try to avoid to ride with this big size any way they can - so they prefer to be slightly underpowered on a 6-7-8m2 instead of riding with such a slow jumbo
This goes for spots with changing winds or low pressure areas, where you can wait for the right conditions - and particulary at wind driven wavespots.
But of course, at some spots where the wind is too "steady" and low, you havent got a choice
Still - as soon as you've learned to ride your Wam, you'll definitely need a 6 or 7m2 (especially when Evos) in 20-25 knots of wind, no doubt at all Beppe - when 70kg.