There is a limit to the file uploads in WebGUI.  If you need to temporarily upload large files, it is recommended that you set the limit high enough, upload the file, then reset the limit. The limit is the TOTAL size of all the files you are uploading at one time. So if the limit is set to 10M, and you need to upload 11 files at 1 M each you will exceed the limit.

For those who do not like doing math: http://www.webmasterknowhow.com/html/bytecompare.html

If you are trying to upload big files, but don't succeed (the user may not get any error messages), you can check two things:

1. Make sure the Maximum attachment size is set high enough. You can find that setting under Admin console > Settings > UI (tab) (default is 10000 which is 10Megs)
2. If that setting is correct and your big uploads still fail, please check the apache error log of your virtual host. If it says something like:
   

   Error string not specified yet: Content-Length header (98556532) exceeds configured max_body limit (67108864)

   the the default read_limit variable of libapreq is set too low. (by default this is 64M). To increase it set it to a higher value by adding the following line to your httpd.conf (or modperl.conf if you are using wre):
   

   APREQ2_ReadLimit 1024M

   In this case uploads up to a gigabyte will be accepted by apache.

Please note:

1. The  APREQ_ReadLimit will not override the Maximum attachment size option.
2. The 1024M setting is essentially insane because HTTP is not really designed for transfering such big files. But if you don't want to be hindered by the APREQ_ReadLimit it is an okay value.
3. Setting APREQ_ReadLimit really high potentially opens a DoS attack vector. So if you want to be on the safe side, set it to the maximum file size you expect to be uploaded.