At what size is a subchorionic hemorrhage considered large or dangerous.?

All increase risk: Subchorionic hemorrhages are very common and most resolve on their own without causing any problems. Obviously the smaller sizes one to 2 cm are less concerning than a larger. Size alone cannot predict which one will lead to problems like miscarriage and which ones will resolve.

~50% Placental Bed: A subchorionic hemorrhage/hematoma is a bleed between the pregnancy and the uterine wall. It is an US finding, that if coupled with vaginal bleeding, can potentially increase the risk of miscarriage/preterm delivery, but w/out vaginal bleeding, it's hard to say. Size matters; worse if bigger (covering ~50% of placental bed) or occurring later. It can heal itself, but you just have to wait/watch.