Nick Edwards has bad news and a workaround:
Have you ever come across an issue where your Power BI matrix column widths just aren’t the same width and visually just don’t look right?
Unfortunately (as of April 21’) there is no easy way to make all column widths equal in the format pane of a matrix visual.
However there is a hack to set the width of your all columns in a matrix so that they are all equal and pixel perfect with DAX!
Click through to see how. It’s a hack in all the finest ways: clever, unexpected, and a bit unwieldy.