Hi HomerJ,
The max file size is determined by 2 things:

1) the setting on the following parameter:
UPLOAD_ATTACH_MAX_SIZE
Description This parameter prevents the upload of attachments whose size is greater than the integer value specified for this parameter. The size is in kilobytes. Note: This parameter does not affect uploads of attachments in integration tools. The default value is -1 (meaning, all attachments are uploaded regardless of size).
Default value -1
Min value -1
Max value 9223372036854775807
Introduced in release QC 10.00
Introduced in patch GA
System false

As you can see the default setting for this parameter is unlimited and is what the installation reverts to, unless you add this parameter to the site configuration tab within the site administration and give it a maximum value size. In the absences of this parameter within the list it allows all attachments no matter the size to be uploaded and then defaults to the next determining factor:

2) the amount of free space on the repository location and within the DB. So long as the default setting of unlimited is set then the limitations of your free space available to store the attachment becomes the limiting factor (and this could be limited by the OS that is hosting the server software too).

Hope this clarifies things,
Dan