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 font is Calibri, select your preferred Font from the drop down list. Examples to try:
Gadugi / Bahnschrift Light / Microsoft YaHei UI / Segoe UI