class DifferentlySizedTextarea(forms.Textarea):

    def __init__(self, *args, **kwargs):
        attrs = kwargs.setdefault('attrs', {})
attrs.setdefault('cols', 80)
attrs.setdefault('rows', 5)
super(DifferentlySizedTextarea, self).__init__(*args, **kwargs)


class MyModelAdmin(admin.ModelAdmin):
    pass
Resize fields in Django Admin