"Het is een grote windmolen."
Translation:It is a big windmill.
That's right, in Dutch this is molenaar.
BTW, although both versions are windmolens (watermolens are rather rare in the Netherlands) I would always simply use molen for the traditional windmills, which used to grind grain into flour:
Windmolen is more common for the newer version generating electricity:
These (modern structures) are often called windmills in English too, though, more correctly (since they do not "mill" - i.e. grind - anything), they are wind turbines.