Q. Mishna Berura (225: 20) writes that you make the bracha of meshane habriot (who created assorted creatures) when you see an extremely tall or heavy person. How tall and heavy do they have to be? How about a very tall basket ball player or very heavy Sumo fighter? Do you say the bracha with Hashem’s name? Do you say it each time you see them?

A. On question 551 in regards to reciting the “meshane habrios” brocho on a Platypus we wrote that; Shulchan Aruch (O.H. 225: 8) rules that one recites the brocho of “meshane habrios” when seeing a truly unusual creature, (then) such as a monkey or elephant for the very first time. Poskim write that any other truly unusual or abnormal creatures qualify too (Mekor Chaim, see also Piskey Teshuvos (225: 21, Halacha Berura 225; Yalkut Yosef 3 p. 608). The blessing is said when one is genuinely amazed and astonished at the human or creature one is encountering (Piskei Teshuvos ibid. 23). Since this would depend on the eyes of the beholder and one’s life history and experiences, it is hard to give exact measurements. (Tallest basketball player is reported to be over seven and a half feet, while Sumo wrestler over 265 kg.).
Shulchan Aruch (225: 9) presents a difference of opinion between the Mechaber who rules to make the brocho only the first time one encounters the unusual, and the Rema, who maintains it can be said after a thirty day lapse. Mishna Berura (ibid. 30) opines to recite it without Hashem’s name, after thirty days.
However, other Poskim agree with Rema, that after thirty days a brocho with Hashem’s name could be recited when encountering a different subject with similary unusual characteristics. (Eishel Avrohom ibid. 14, Shulchan Shloimo ibid. 7, Aruch Hashulchan ibid. 13. Tzitz Eliezer 12: 22, and others).
Horav Shlomo Miller’s Shlit’a opinion is that whenever, there is any doubt on having to recite a brocho, one should abstain from doing so.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a