http://chriskief.com/2013/10/19/limiting-upload-file-size-with-django-forms/

fields.py

from django import forms
from django.template.defaultfilters import filesizeformat
from django.utils.translation import ugettext_lazy as _
from django.conf import settings

class RestrictedFileField(forms.FileField):
    def __init__(self, *args, **kwargs):
        self.content_types = kwargs.pop('content_types', None)
        self.max_upload_size = kwargs.pop('max_upload_size', None)
        if not self.max_upload_size:
            self.max_upload_size = settings.MAX_UPLOAD_SIZE
        super(RestrictedFileField, self).__init__(*args, **kwargs)

    def clean(self, *args, **kwargs):
        data = super(RestrictedFileField, self).clean(*args, **kwargs)
        try:
            if data.content_type in self.content_types:
                if data.size > self.max_upload_size:
                    raise forms.ValidationError(_('File size must be under %s. Current file size is %s.') % (filesizeformat(self.max_upload_size), filesizeformat(data.size)))
            else:
                raise forms.ValidationError(_('File type (%s) is not supported.') % data.content_type)
        except AttributeError:
            pass

        return data

class RestrictedImageField(forms.ImageField):
    def __init__(self, *args, **kwargs):
        self.max_upload_size = kwargs.pop('max_upload_size', None)
        if not self.max_upload_size:
            self.max_upload_size = settings.MAX_UPLOAD_SIZE
        super(RestrictedImageField, self).__init__(*args, **kwargs)

    def clean(self, *args, **kwargs):
        data = super(RestrictedImageField, self).clean(*args, **kwargs)
        try:
            if data.size > self.max_upload_size:
                raise forms.ValidationError(_('File size must be under %s. Current file size is %s.') % (filesizeformat(self.max_upload_size), filesizeformat(data.size)))
        except AttributeError:
            pass

        return data

Hi. This is for forms, thus doesn't show up in admin when used. You've got link to snippet for Models (to be able to show in admin) instead of Django forms?

Update: Got it working. Instead of subclassing forms.FileField, I subclassed models.FileField
See fork for update