Hi Teddy,

re:
> I'm not sure why the size of the plane mattered, but it did. Good to know.

On your original file, the plane is just barely not quite extending all the way across the object, there is a gap of about 0.001 units or so on one side, you have to zoom in quite a ways in the Back view to see it though, but it looks like this:





So because the plane does not fully cross the object on that one side, it does not fully divide the object into 2 pieces when you do the boolean.

If you widen it just a bit like this then it will work since it fully divides the object then:




It's generally a good idea to make things like cutting planes extend a little ways outside of the object that is being cut just to make sure that it fully crosses the object, if it does not fully cross the object the boolean won't cut the object into pieces like you want.

- Michael