Since nobody else knows, I've found the answer myself.
In equation editor open up a matrix. By default this is in "professional" mode, so it looks like a matrix.
Right click the matrix area and click "Linear". This turns it into a string of &&@, where & is the matrix cell placeholder and @ indicates the last cell in a row.
So just add more &'s and @'s to increase the size.

Easy enough once you've figured it out, but a bit of help in the "help" would be nice please Microsoft!