I'm afraid there isn't such a thing as "a valid signed value", if we're still talking about "size wise".
For django (python), the integer you gave in the ticket is perfectly valid. Here's a way for you to check that :
>>> s = '351760125423456632454565345363453423453465345453'
And indeed, an IntegerField validates that the content of the field can be converted to an int this way (check django/forms/fields.py line 230).
So definitely, as Alex pointed, this is an issue on MySQL's side, not Django's.
I believe this can't (shan't?) be fixed at Django's level, as there's no "size" limitation for the IntegerField, as you would have on a CharField with the max_length
And no, limiting the length of the string won't work, as "2147483647
" isn't the same length as "-2147483647
", but is the same length as "9999999999" (if we're taking the example of 2^32-1 as the max SIGNED INT value).
my two cents ;)