You cannot pick a font size, as the report cells are dynamic and the size of the font will reduce / increase to fit the data to the cell.
However, what you can do is to CHANGE the Font, to something that will give you a larger font in general to see if that helps.
Do this in the Report Styles. Calxa Original is Calibri, select your preferred Font from the list, or try the following: Gadugi / Bahnschrift Light / Microsoft YaHei UI / Segoe UI