You can setup the font size you preferred as default so whenever you open a workbook, you have the font size you want, please check How to Set the Default Excel Font Size for details.
At the same time, you can change the font size anytime with the steps below:
Step 1: Select the cell or the data range you need to change the font size;
Step 2: Click the "Home" Tab from the Ribbon;
Step 3: Click the font size box and select the font size from the drop-down list, for example, "16";
Step 4: You will find the font size is now different;
Step 5: You can always type the size of the font you want in the box, e.g., 56.