If you get used a certain font size and you want to have that whenever you open a new Excel workbook, you can set this font as the default Font size. Otherwise, please check How to Change Font Size within the worksheet.
To set the default font size (e.g., 11), please follow the steps below:
Step 1: Click the "File" Tab from the ribbon;
Step 2: Click "Options" from the left navigation menu;
Step 3: Click "General" from the left navigation menu in the dialog box;
Step 4: Select your favorite font size from the drop-down list of "Font size" under the section of "When creating new workbooks", for example, font size of "11";
Step 5: Click "OK" at the bottom.