HFSS Lumped RLC Boundary

Hello, apparently this is a common question here, but i haven't found a final answer. How can i determine the dimensions of the rectangle that is going to be used as an RLC boundary? I'm trying to replicate a paper, but when i do the rectangle really small, i don't match with the results from the paper. I have to play with the width till i get into a sweet spot. Anyone knows how's the correct way to choose the size?