I assume you're using the combo box available from the control toolbox,
not the forms toolbar.
Make sure the control toolbox is visible, select the combo box and
click on the properties icon (it's the one with a picture of a hand
pinting at a list). The properties window will come up. On the left
hand column of it, find the word Font. Click on it, and you will see a
small button with three dots appear. Click on that button. A dialog box
will pop up, where you can change the font size of the combo box. Click
OK.