As exposed in MixedForm test in django-select2 source code, options given to select2_options are not used because width is not an option known of django-select2 and because select2 expects placeholder not placeHolder.
class FooSelect2Field(django_select2.AutoModelSelect2Field):
queryset = Foo.objects
search_fields = ['name__icontains, ]
station = StationSelect2Field(widget=django_select2.AutoHeavySelect2Widget(select2_options={
'width': '32em',
'placeholder': _(u"Search foo")
})