I've had a few wing lessons and am now looking to make my own way with some secondhand stuff, firstly on a windsurf board with dagger and then when I can afford it, a wingfoil board. I'm mainly at a lake with wind about 10-15 knots. I weigh 70kg and have just got myself a 6m wing - is that about right, or too big?

Yes that's fine

A bit too big IMO

It's a bit hard to tell. I'm 70 kg and 175cm tall. I found a 5.5m Slick was a touch too big as the wingtips kept touching down when I pump onto the foil. A 5m Slick is perfect. For me, wingspan is an important measure. I think the threshold for me is around 3.1m tip to tip.
So area isn't necessarily the most defining measure.

I'm 186cm 85kg, I have a 6.5, 5.5 and a 4.5 all slicks, 6.5 for me requires at least 12knts of wind with my HPS 1050, I am lazy and go for passive take offs.But I will soon be buying a 8m Strike CWC for those summer 8knt days and may be ditch the 6.5m