What effect does a field name length has on the performance of a database? Is it negligible? Should I go with long descriptive names?
I haven't measured the time, but I would imagine negligible. The field names are going to be sent back and forth over the network, both in your SQL statements TO the database and in the metadata that comes back FROM the database, but I think the incremental difference that a few bytes makes in those packets is below the threshold of caring.
Pity the poor humans that have to read your code, and make the names descriptive. Let the computer to a tiny bit of extra work.