Field in the model does not change in django app

I'm working on a django 1.4.5. I added to the model, select the size of T-shirts.

Field in my model

tshirt_size = models.CharField(choices=TSHIRT_SIZE_CHOICES, default="s", blank=True, null=True, max_length=24)

My form

class SubscriberForm(forms.ModelForm):
    class Meta():
        model = Subscriber
        exclude = ['event', 'is_active']
        widgets = {
            'name': TextInput(),
            'last_name': TextInput(),
            'email': TextInput(),
            'tshirt_size': Select(choices=TSHIRT_SIZE_CHOICES)
        }

In view, I get data in this way:

tshirt_size = request.POST.get('tshirt_size')

Part of html code

<label for="id_tshirt_size">T-Shirt Size (Unisex):</label>
{{ form.tshirt_size }}

And when I execute save on form I get in admin panel None value for tshirt_size.