Style the Listbox element inside of a Form control to increase its height

Whatever you are using the form control on, page type, form, etc, in the fields list where you select the control, you should be able to add a control css class, then you can just style the control based off that.

You can access it via css using the following selector:

.form-group .editing-form-value-cell select.form-control

You can also add a value to the "Control CSS class" within the field of the object (page type, custom table, form, etc.) you are working with. This will add the css class you added directly to the input control so you can select it easier.