According to the My Hero Academia Wikia Mount Lady can enlarge to 20,62 metres. In the other hand, it's shown in the anime Yui Kodai can also enlarge herself. Is it known how much can she enlarge? Who has the most powerful enlarger quirk, Yui Kodai or Mount Lady?
We don't know how much Yui can enlarge herself. However, there are two distinct points that make their quirk different.
Mount Lady only enlarges herself to 2062 cm. That's it. However, Yui can change her size as she wants. If her quirk has no limits (again we don't know) she might overgrow Mount Lady.
Yui also shrinks herself but that's not relevant for the question.
Anime series watcher here, didn't read the manga
As JustPlain said in his answer, we don't know how much Yui can enlarge herself. But there's one thing you have to consider about the "My Hero Academia" universe: Nothing is given for free. As powerful as some quirks might be, they all require something to be used/consumed in order to work.
Momo Yaoyorozu and Rikidou Satou both need to eat a lot to use their quirks to their best.
Ochako Uraraka needs more and more concentration and strength as she makes float more and/or larger objects.
Izuku Midoriya and Eijiro Kirishima both need energy to use their quirks; the more tired they get/the more they use their power, the less powerful their next attack will be.
As it appears right now, every character in the show has a mental or physical threshold they can't go past. Given the fact that Yui Kodai can both grow and shrink at will and that she's most probably less experienced with her quirk than Mount Lady is, I would say that for the moment being she can't overgrow her.
Even in the future it'll probably be hard for her to overgrow Mount Lady since her quirk "grow" threshold is probably tied to her "shrink" threshold, meaning that she'll have to either chose to practice one and not the other, or practice both but get less impressive results out of them.