I'm assuming a few things here.
One, that you're using a combobox that's planted directly on the sheet and two, it's from the Forms toolbar.
If these are correct, try using the combobox from the Control Toolbox toolbar. You can then set the font to any size you like in the font property.
Ron's steering you in the right direction.
While in design mode, right click your ComboBox, choose Properties and then scroll down the properties window. When you see the property "ListFillRange" simply enter the range of your list.
Once you're done setting the font properties you want, you should be good to go.