Short Text Conversation (STC) is a new NTCIR task which tackles the following research question: given a microblog repository and a new post to that microblog, can systems reuse an old comment from the respository to satisfy the author of the new post? The official evaluation measures of STC are normalised gain at 1 (nG@1), normalised expected reciprocal rank at 10 (nERR@10), and P+, all of which can be regarded as evaluation measures for navigational intents. In this study, we apply the topic set size design technique of Sakai to decide on the number of test topics, using variance estimates of the above evaluation measures. Our main conclusion is to create 100 test topics, but what distinguishes our work from other tasks with similar topic set sizes is that we know what this topic set size means from a statistical viewpoint for each of our evaluation measures. We also demonstrate that, under the same set of statistical requirements, the topic set sizes required by nERR@10 and P+ are more or less the same, while nG@1 requires more than twice as many topics. To our knowledge, our task is the first among all efforts at TREC-like evaluation conferences to actually create a new test collection by using this principled approach.