I previously posted about this topic HERE and explained the position of Rav Moshe who assumes it is the size of a pinkie finger making the diameter to be about 7mm (and maybe even 1cm which is 10mm).
Rav Moshe says that aside from the Mishna in Ohalos which gives an unfamiliar shiur and assumes that it would be larger than the size of one's pinkie. However, in our gemara it discusses a woman who miscarries a 40 day old fetus as a dry birth which doesn't render her tamei. The gemara asks that the opening of the uterus should definitely cause some blood, to which the gemara concludes that it must be אפשר לפתיחת הקבר בלא דם. This would be the source of the Rambam to reject the entire notion of אי אפשר לפתיחת הקבר בלא דם, because the gemara would clearly be saying that it is possible. However, the gilyon hashas quotes an alternate girsa from the Rashba. The gemara thought that even the opening of the uterus caused by a 40 day old fetus should be considered an opening of the uterus for the purpose of assuming there will definitely be blood, to which the gemara responds that since the fetus at 40 days is so small, it won't cause bleeding. The girsa of the Rashba actually states clearly that the size of the fetus at 40 days is exactly the shiur for this assumption of אי אפשר לפתיחת הקבר בלא דם. According to ultrasound measurements that we are privy to nowadays and scientific estimations, a fetus that is between 5 and 6 weeks from conception will be between 2mm and 4mm in size. Therefore it should follow that any time a physician inserts an instrument with a diameter larger that 4mm into the uterus we would have to say אי אפשר לפתיחת הקבר בלא דם and render her tamei. This is a big chumra because it reduces the shiur of R. Moshe by half and seems much clearer that trying to estimate the shiur referenced in the mishna in ohalos.