Matching mutable object without ArgumentCaptor

I have to test a method which uses a mutable object

private final List<LogMessage> buffer;
...
flushBuffer() {
  sender.send(buffer);
  buffer.clear();
}

I need to test that it sends buffers with exact size.
ArgumentCaptor is not applicable because the captured collection is clear by the time of assertion.

Is there a kind of matcher which can reuse Hamcrest's hasSize() and does check right in time of method call?

I would prefer something like this hypothetical collectionWhich matcher:

bufferedSender.flushBuffer();
verify(sender).send(collectionWhich(hasSize(5)));