I think you only get this error when downloading a GCD file. If you are downloading a JAD file and there is a size mismatch, you may get a 905 error instead.

Make sure that the file size specified in the GCD matches exactly the size of the file you are trying to download (ringtone, picture, whatever).

Don't use commas in the size.
The size is the number of bytes of the file. It is not the "size on disk" of the file.