public class MyRetryPolicy implements RetryPolicy {
    private static final int MIN_FETCH_SIZE = 1000;

    @Override
    public RetryDecision onReadTimeout(Statement statement, ConsistencyLevel cl, int requiredResponses, int receivedResponses, boolean dataRetrieved, int nbRetry) {
        if (receivedResponses < requiredResponses && statement.getFetchSize() > MIN_FETCH_SIZE) {
            statement.setFetchSize(Math.max(MIN_FETCH_SIZE, statement.getFetchSize() / 2));
            return RetryDecision.retry(cl);
        } else {
            return RetryDecision.rethrow();
        }
    }

    ...