They’re big enough to be pets. Hell, they are bigger than my pets.
They are looking for caterpillars to take home and lay eggs in. They may fly up and down your legs and arms, scanning for worms… but they normally don’t atack unless threatened. If they do attack, it can be extremely painful, like a bad scorpion sting. Much worse than a bee sting. Multiple stings can be fatal.
The large red wasps here have been known to kill people, such as two years ago occured with a group in the Red River Valley near WuLai, and other groups hiking in other further afield ranges that summer… I would guess that the nest was very close to the trail, and that perhaps someone threw stone at it or otherwise disturbed it. Be aware that these wasps can be dangerous but generally are not if you leave them alone. Don’t swat them away! Not using perfumes or floral smells is known to be effectve for the European wasps, Vespa vulgaris, as adult wasps will often visit flowers to get sugar water for their own flight energy, which is also why you sometimes find them drowning and hence very angry in your soft-drink can. I am not sure if the same goes for the wasps in Taiwan, most of which are NOT vespa vulgaris.