does enayone knows how to calculate theHarrowed Realm's size?
I can't access that website and don't know what the Harrowed Realm is, but assuming it's a plane it's probably infinite in size.
"The Harrowed Realm is a demiplane originally created from the Dimension of Dreams by the Varisian bard called Sonnorae. It is, however, now sealed off from all planes except the Material Plane, to which it is connected through the Deck of Harrowing Tales"
Harrowed Realm
(Plane)
Sphere Inner Sphere demiplane
Alignment Mildly Neutral
Denizens Storykin
Description Small, ageless demiplane
It's from the Pathfinder Module: The Harrowing
Well, there is no calculation possible for determining a plane's size. You could calculate how big a single casting of a the spell Create Demiplane would be at the appropriate caster level and such, but have no way of knowing how many times it was used.
Simply make the plane as big as you want to accommodate the story.
Thanks for the advice!