Hi Simon,
Even if you are light weight, a 12m kite size makes perfect sense for 10-15 knots winds.
Many other factors need to be taken in considerion as well. As mentioned by Sandee, check with other people in your local area who will give you better advice based on your local conditions.
You need to consider other factors such as as wind density and consistency, shallow water vs deep water, etc.
Like most kiters, you will eventually end up getting multiple kite sizes. Your next size should be something like a 9m kite for winds above 15 knots.
Have a look at our blog article and kite size calculator :
www.kitebud.com.au/how-to-choose-a-kite-size/As you can see it already suggests a 12m kite for your weight and wind conditions.

If you slide the wind speed from 12-22 knots for example, you will see it suggests a 9m + 12m kite.
Hope this helps.
Christian - KiteBud