When you create a node with the designer, an item is added to the Style Class list automatically, two in the case of the combo box. They are listed in light gray text. You can access the classes with a '.' so on a new line in the style sheet put:
.combo-box (-fx-font-size:20;}
or whatever css style you want. You can add multiple classes to the Style Class and make your own groupings, just add the same class name to multiple nodes, and they will all get the styles you declare for the class.
You can also access the items via the ID with a '#' so:
#combo1 {-fx-font-size:20;}
should also do it. but as there are two classes generated, I'm not sure which element of the combo box will get the style possibly both.
Be careful of this one if you are capturing input from a number of TextAreas or other nodes using the sender, as they will need to have the same id. It may be simpler to stick to classes.
If you haven't already seen it, the javafx css reference is
here.