Hello @kartik,
For Django >= 1.8 you can use the Length function, which is @Pratyush's CHAR_LENGTH() under the hood for MySQL, or LENGTH() for some other databases:
from django.db.models.functions import Length
qs = MyModel.objects.annotate(text_len=Length('text_field_name')).filter(
text_len__gt=10)
Hope it helps!!
Thank you!