In episode 177, "Reunion with the Black Organization", Conan asked Haibara to drink the liquor which made her restore to her original size, but why did not he tell her that the first time he met her or at least after he trusted her? This is a very important information which cannot be delayed.
Why did not Conan tell Haibara about the liquor that made him restore to his original size as soon as possible?
1I'm not sure why he kept it from Haibara, but at that point, it's very likely that he still didn't trust her. Haibara first appearance in manga is in chapter 176 and he told her about the wine in chapter 241. That's only 65 chapters apart, and in those 65 chapters there are only 5 cases of which that include Haibara. From Conan point of view, it was probably not enough time to trust her.– DarjeelingNov 5, 2017 at 4:48
But even if he didn't trust her, I don't think there's a reason to kept it secret. They even want to find the drug data and hoping to create the antidote when Conan know that Haibara is the person who make APTX 4869. Unfortunately the data is gone so they can't make the antidote.– DarjeelingNov 5, 2017 at 4:48
Remember that once your body develops an immunity against baiju thus you can't grow twice. Conan was probably thinking that it might be useful in the fell clutch of circumstance.
Plot-wise, if Haibara already drank baiju and so, if Haibara wasn't in adult form, Conan and Haibara will be unprotected and might die. Since they have this plot immunity, then that won't happen.