The next version v15.5.0 will be released soon, and it will include a registry value. Also, the default value becomes 80 (it was 40 in older versions).
The following REG_DWORD values can be set in the HKEY_CURRENT_USER\SOFTWARE\EmSoft\EmEditor v3\Common key to set the maximum length for the file name when the Automatically Name Untitled Document check box is checked in the Save Details dialog box.
MaxNameUntitled: the maximum length for the file name including the terminating NULL character. Default: 0x50 (80)