Oleksandr Bogonis is a Windows-to-OS X switcher who generally likes the move, but he’s frustrating by a limit he seems to have run up against:
A field for a file name in “Save as...” box is critically short. It allows one to enter 30 or so symbols. I often save files which have long names—up to 100 symbols.
He wonders if there’s a trick to increase file-name length. The real limit is 255 characters, so something is definitely off.
It could be one of a few scenarios:
You’re trying to save out of Apple’s Mail app and it has a corrupted index. Many users across many forums and years report being told the character limit for a volume is 31 characters, then re-indexing Mail, and the problem disappears. Go figure! But if it works, it works. The simplest way to fix Mail is to re-do your whole Spotlight index per Apple’s instructions. (There are outdated instructions on Macworld and elsewhere for deleting Mail’s index by itself, but these seem unreliable in El Capitan.)