To increase the font size in formula bar, you need to go to the Excel Options . Steps as follows: 

STEP 1

Click File > Options to go to the Excel Options dialog box, then click General from the left pane, and then choose the font size you need from the Font size drop down list in the right list box, see screenshot:

STEP 2

Then click OK to close this dialog, and a prompt will remind you to restart Excel for the changes to  take effect. Once you reopen, the new font size will be rendered in the formula bar