Huge - up to 2m tall - and unmistakable.
Apparently the dried stalks are the best thing for fire-by-friction using the hand drill method. I don't know what the best baseplate is for that but seem to recall ivy was good, if you can get a big enough piece? Maybe someone else can remember better.
I need to find a good place where there's a bunch of mullein growing and collect a hundred dead stalks, finally learn hand drill fire starting. Bow drill is reliable enough if you've got the stamina, you'll basically get there eventually. But I hear hand drill is a massive pain.