Castellan Toad Boss' physical hitbox.

All bosses that extend their range have this. (Halloween Toad, Cat, etc)

You have to stand about 25 pixels from the portal and it won't hit you with that attack.

I'm sure there's more safe area than that. The tongue attack is actually magic, and I'm sure that I can survive it. However when it uses that attack, its physical touch hitbox also extends outwards - just not the whole length of that attack so if I'm close enough I take touch damage, but if I'm not I take magic damage. What I want to know is how far that hitbox extends, so I can find out how to position myself.

It can change direction, you have to take a touch hit first, etc.

It can't change direction, but if I happen to find it already transformed, it's impossible to get over it without taking touch damage.