Multiple select widget does not honor size attribute

[Jonah Bishop]

I have an admin page set up for a model like so:

@admin.register(p_models.ScheduleType)
class ScheduleTypeAdmin(admin.ModelAdmin):
    formfield_overrides = {
        models.ManyToManyField: {'widget': forms.SelectMultiple(attrs={'size': '30'})},
    }

    list_display = ('name',)

The resulting ManyToMany field has the size attribute set properly, but every browser I try only shows about 8 rows (not 30, as I instructed). I've cleared cache, and tried in multiple browsers (Chrome 63, Firefox 57). My site uses Django 1.11.9.

It looks like the CSS style rules are overriding the size attribute. I'm guessing adding a height: auto; rule to the select[multiple] style set would fix it (it appears to do so in Chrome, at least).