Cannot use more than 1 collision block per square

[Rinelw]

I cannot use more than 1 collision block per tile with your map editor. RMMZ 1.5.0

[Hudell]

You just need to set the "Default Collision Blocks" parameter to a different value (2 or 4) and it will show smaller blocks to customize the collision.

But for this to have an actual effect in your game, you'll have to use a plugin that reads this collision data. Cyclone Movement will do this automatically, but if you're using parallax mapping you can also export the collision layer to an image and some other movement plugins may support reading the data from that.