I built a forecasting solution for a client in Excel. Told them they needed a database, they said "Yes, but we need a spreadsheet NOW", so for teh moment that is the solution. Database in gestation...
The biggest file in that system get to 55Mb and was unstable as hell. I reorganised things so now the biggest files get into the mid to high 20Mb range. They work OK, but you need to have calculation set to Manual unless you like watching paint dry, and stay away from external links.
Like Joe said, Excel has memory limits which you would do well to avoid. If you are effectively using the workbook as a database, why don't you push the data to Access, then pull back the stuff you need for reporting? You can have a whole bunch of files pushing data to the DB, then have a reporting file extract filtered data for you to play with.
Denis