There are only three beings I know of in the MCU that are big, but I can't tell which one is the biggest. Which one of these being is biggest, and is there anyone bigger than that?
Dormammu
Knowhere (or rather, the Celestials when they were alive)
Ego the planet
Currently, the largest being whose size was shown in MCU media is Arishem (a Celestial).
In his appearance in Eternals, he is revealed to be approximately the same size as the earth. For comparison, Ego's planet is "no larger than [...] earth's moon" according to Ego in Guardians of the Galaxy Vol. 2.
Also shown in Eternals is Tiamut (another Celestial). His form was revealed as a vision to Sersi by Arishem. He is also shown to be around the size of the earth.
Considering the options you gave, the answer would probably be Dormammu. However, his appearance in Doctor Strange only appears to be a reflection of the titular character himself, so his true corporeal form may be undefined.
Given that, another answer up for consideration would have to be Ego, for obvious reasons. He is a planet, after all. However, considering the fact that he is a Celestial, there may be others of his species that are even larger. I suppose we won't know until Guardians of the Galaxy Vol. 3.
The largest character is The Living Tribunal he has to power to increase his size an infinite amount. Basically, he can be bigger than every other character if he wants to. The second biggest and third biggest are the Oblivion entity and the Eternity entity. Their sizes are unknown but they are still bigger than everyone besides The Living Tribunal.
