Dynamic length Django model field
No you can't. The length of string that can be accepted relies on the structure of your database -> you'd have to migrate your database on every max_length change! The solution to your problem would be to simply measure a potential maximum size of variable you'd like to save (here I think about some dev environment). Then set max_length accordingly.
I have one Django model field definition like this:
landMark = models.CharField(max_length=50, blank=True)
Can I define the landMark field dynamically so that it is capable of holding a string which has a varying size ?