I think that excess caffeine would probably be bad , but if your working in a night shift or something , it’s better to drink it then be tired and washed out which has stronger negative effects.
Also I heard it’s a good idea to “cycle” it.
To keep yourself sensitive to it.
As much as flaccid size goes , it think it can temporarily reduce bloodstream to the penis as it works by taking all the blood from the peripheral areas \ organs and “activating” the CNS (nervous system) muscles.
So it might hinder gains because your dick doesn’t metabolize that well , although if you don’t notice immediate changes (when it takes effect o your body) , it won’t do anything.