Can a DBA override the largest value a datatype can have bigint (making it less than what the documentation says)?
bigint
Yes, you could set a check constraint on a column
Example
ALTER TABLE SomeTable ADD CONSTRAINT chkMaxValue CHECK (SomeCol < 123456 ); GO
You can also use a trigger to constrain it, but this is overkill.
no, but you can create a check yourself so that the values do not exceed a certain value, for example:
create table test_bigint( my_value bigint check (my_value <100) )