We generally counsel patience (
PatienceIsAVirtue) here at the Ranch. However, I can understand how you'd get antsy after three days with no replies.
One solution is to stream your objects into memory using a ByteArrayOutputStream. That will let you get your serialized object as a byte array, so you can see how big it is. Then you can stream the byte array out to your file. Another solution would be to record the current size of your file, stream the object to it, then truncate the file back to the original size if it got too big.