I recently started watching Detective Conan and I learned that Ran and Shinichi are in fact childhood friends. So my question is, when Shinichi, due to the side effects of the drug he was forced to take, end up with the appearance of an elementary school student, why did Ran, who has known him since childhood not see the resemblance right away?
She may well have noticed the resemblance, but seeing an elementary school student who looks a lot like how a friend of yours used to look in the past, does not immediately lead one to the conclusion that that student is in fact your actual friend! It is never explicitly stated, but in all likelihood she simply noted (subconsciously?) the similarity (if she were paying attention in the first place) and moved on.
Ran had a few suspicion arcs, where she believed (Desperate revival) or suspected Conan's true identity. The truth is quite nonsense, if someone comes and says your friend has shrinked from 17 years to 7 years, would you believe? It is very illogical and hard to believe. Plus Conan always fools her into some kind of trick at the end of suspicion arcs and he has a lot of other people to help him out (Professor, Heiji, His parents, Ai), and moreover Conan tries to act like a kid around her and wears big glasses. So it shouldn't trigger more suspicion than we see. She just thinks his big resemblance to Shinichi is some kind of coincidence, though she suspects at times, and there shouldn't be more suspicion than this logically.