The gemara quotes the Mishna in Ohalos (7,4) which says that the shiur for pesichas ha'kever is עד שיעגילו ראש כפיקה. The gemara explains that it is the size of a פיקה של צמר which is used for the שתי stitch. The context of the mishna is to know at which point the cervix is considered open enough so that the dead fetus inside can no longer be considered טומאה בלועה and will be me'tamei the vessels that are in the same room as her. It is unclear if this is also the shiur for פתיחת הקבר in the context of the statement in nidah - אין פתיחת הקבר בלא דם. The Nodeh B'Yehuda cited by the Pischei Teshuva (Y.D. 194,4) says that even forcing the cervix open from the outside would qualify as an opening of the cervix and we would have to be concerned that some blood was let out of the cervix. R. Moshe (Igros Moshe Y.D. 1, 83) says that although the binas adam and chasam sofer disagree with the nodeh b'yehuda, they only disagree when one would use a finger to open the cervix because they assume that a finger wouldn't reach through the cervix. However, even they would agree with the Nodeh B'yehuda that if a tool or instrument is used to enter the cervix, it could qualify as an opening of the cervix where we would say אין פתיחת הקבר בלא דם. R. Moshe (Igros Moshe Y.D. 1, 89) says that although the Nodeh B'yehuda says that only something with a very small diameter such as a pieces of straw wouldn't be considered an opening of the cervix, implying that if it were slightly larger it would be considered an opening of the cervix rendering her a nidah. R. Moshe disagrees because the only shiur that we find in chazal for the opening of the cervix is the shiur of כפיקה של צמר, which they also term as a פתיחת הקבר, therefore it is logical that this would be the shiur also in the context of אין פתיחת הקבר בלא דם. Although it is difficult to know exactly the size of the פיקה של צמר that is used for the שתי stitch, R. Moshe assumes that it must be smaller than the diameter of a small finger. Being that the Rambam paskens that it is possible to have an opening of the cervix without any bleeding, and it is possible that an externally forced opening wouldn't qualify as a פתיחת הקבר, R. Moshe is confident in being lenient with any instrument where the diameter is less than the size of an average persons pinky finger.