Make the text inside the Left and Top borders of worksheet larger

In one of my Excel 2016 worksheets the left-hand border (the one that has all the row numbers going down the side) has extremely small row numbers.

I would like to make the numbers bigger and easier to read without making the data inside the worksheet larger and without changing the row heights.

I'm sure this is something simple but I just can't find the answer and would appreciate the education.

Many thanks