I have a forms.models.ModelMultipleChoiceField using an autocomplete.ModelSelect2Multiple widget.
This is working well, except that when I add many entries and it spans multiple lines, the box does not expand to accommodate them (see image below).
Can anyone please suggest the best way to make the box expand?

OK, this turned out to be much easier than I thought. Whoops.
For .custom-select, .select2-selection.custom-select set: