How to change the size of mp_grid?


I'm making a top-down point and click game and I need to use mp_grid, since mp_potential isn't doing as I wanted. (it gets stuck too much)

I've read the manual and blog posts about it, and I think I got how to deal with other issues I was having.

But here is a problem I couldn't solve with it :
not every room have the same width and height, and I don't know how to change the cell size.
should I destroy the grid in the room end and make a new one in the room start event?

I'm using gms2 version 2.3.0 - IDE and for runtime, and haven't made the code yet(I'm asking before I mess up).