Go to Central Administration -> Configure Site Quotas and Locks -> Manage Quota Templates or Manage Site Collection Quotas and Locks. The sizes  are specified there.

Checkout the following from the WSS SP2 Admin Guide:
Configuring Large file  Support
The biggest change included in Windows SharePoint Services SP1 is the support for uploading large files. By default, the maximum size  for uploading files  is set to 50 MB. If you need to be able to upload larger files (such as when you use smigrate.exe to migrate a site between servers), you can change this setting to any value up to 2 GB (2047 MB).

To configure large file support you must increase the default upload size in SharePoint Central Administration. You may also need to perform the following additional actions (depending on your hardware configuration):

Tune the Microsoft Internet Information Services (IIS) connection timeout setting.
The default timeout for connections in IIS is 120 seconds (2 minutes). Depending on your maximum file size and how long it takes for the file to be uploaded, you may not need to change this setting. If, however, IIS is timing out when you upload large files, you can change this property to ensure that larger files can be uploaded successfully.

Increase the default chunk size for large files.
The large-file-chunk-size property sets the amount of data that can be read from server running SQL Server at one time. If you have a file that is greater than your chunk size (such as 70 MB when the chunk size is set to 5 MB), the file would be read in 14 chunks (70 / 5). The chunk size is not related to the maximum upload file size. The chunk size simply specifies the amount of data that can be read from a file at one time. By default, the large-file-chunk-size property is set to 5 MB. If you notice performance or scale problems on the client or server, then you may need to tune this setting to get the performance you are targeting. Note that if you raise the chunk size too high, the files might use up too much front-end memory and you may need to lower this setting.

Increase the maximum size limit for Web Parts.
By default, the maximum size limit for a Web Part is 1 MB. If you need to accomodate large Web Parts, you can change this setting in the web.config file for your server or servers running Windows SharePoint Services.

In theory ("in theory anything is possible"), you have 2GB files (see above). In practice, 50MB to 100MB should be considered on the large side.

What about the maximum upload size? Go to central admin and click on Windows SharePoint Services then Configure vurtual server settings. Select the virtual server and then under Virtual Server Management click on Virtual server general settings. Checkout page 1112 in Microsoft Sharepoint products and Technologies Resource Kit.