In my edition of Office (MS Professional Plus 2016, Windows) I can do:
Option Explicit
Sub AddComboFont()
Dim cb As ComboBox
With ActiveSheet
Set cb = .OLEObjects.Add(ClassType:="Forms.ComboBox.1", Link:=False, _
DisplayAsIcon:=False, Left:=322.5, Top:=11.5, Width:=176.5, Height:= _
61.5).Object
cb.Font.Size = 20
End With
End Sub
Same thing can be achieved with right-click on combobox > properties > Font
This applies to ActiveX
comboboxes. You can't change the Font size on the Form control
combobox. This can be a trade off as Form control can be better behaved than ActiveX particularly when users have different screen resolutions. See my answer here Excel Comboboxes double up on some PCs