Setting column sizes in a matrix

Hi everyone, 

I'm quite new to Power BI so still figuring out how things function on there. I've been trying to set the columns of my matrices to be exact sizes but cannot seem to find an option to do so in 'Format visual'. I've been using the drag bar that appears at the end of each cell of the matrix to resize but when I re-adjust the date of the slider at the top, all the cells of the matrix stay the same size apart from the 'column subtotal' cell at the end which becomes smaller.

Is there any way I could either fix the size of the last cell or set a consistent size across all cells so they don't resize when I apply the slider?

Thank you in advance for any advice!