AFAIK the choices are:
1) write code in your C# application that opens a recordset, assembles
the fixed-length records and writes them to a text file.
2) use the Jet Text ISAM to write the file, either by automating Access
or by writing to the DAO library. Either way you'll need to provide a
schema for the text file. The options are an Access import/export
specification stored in the mdb file, or a schema.ini file in the
destination folder.
The import/export spec is easy to create: start expoirting the data
manually, click the Advanced button in the wizard... but can only be
used with DoCmd.TransferText (i.e. requires you to automate Access
instead of just using DAO). Schema.ini is a bit more obscure, but is
simpler to create on the fly under program control. Because it's
recognised by the Jet Text ISAM, you can just use DAO instead of
automating Access.
Create a Schema.ini file based on an existing table in your database:
http://support.microsoft.com/default.aspx?scid=kb;EN-US;155512
How to Use Schema.ini for Accessing Text Data
http://support.microsoft.com/default.aspx?scid=kb;EN-US;149090
Documentation for Schema.ini is towards the end of the Help topic
"Initializing the Text Data Source Driver" (under Microsoft Jet SQL
Reference).