The practical limit for Monarch to handle on the desktop is about 2GB of data - that's because it uses the JET database engine. In practice, anything that could fit in a Microsoft Access file should be OK in Monarch, and that should allow you to scale up to several million records (depending on how many fields you have, as well as how long they are). Text input reports can be much bigger than this - largely as they tend to consist of mostly white space.
If you need to convert and export more data than this, there are several techniques. If you have access to DataPump, you can convert data directly from the report to a SQL database table, in what we call "table-less" extraction, by-passing the JET engine and Monarch's table window completely. If you only have desktop Monarch, you can throttle the input by using a detail template with literal characters instead of wildcards, and export each section to CSV format.
While the error message would lead one to think that Monarch's trying to load too much into an Access database, between this post and [URL="http://www.monarchforums.com/showthread.php?t=3164"]your first one[/URL] LeeAnn, I'm not so sure that that is what's happening here.
I'm more tempted to suspect that a recent Windows update (or perhaps some other software installation) has modified the MS Jet configuration on your computer.
Have a look at the [URL="http://support.microsoft.com/kb/239114/"]Jet dll version[/URL] (see the section entitled "How to determine the current release level of Jet 4.0") installed in your \Windows\System32 folder and considering re-installing Jet (download and install information is also available on that same web page).
You might want to talk it over with your local IT admin, if possible, as without more information to consider, this is really more of a hunch than anything concrete.