Note that most of the responses you're receiving, while accurate, are in terms of units of dried cubensis mushrooms, not pure psilocybin. A fresh cubensis mushroom weighs about one-tenth its original weight once it's fully dried, and when dried they usually contain about 0.7-1.0% psilocybin by weight. So 2.5 grams of dried mushrooms will contain about 25mg of psilocybin, which is the "25-mg oral dose[] of psilocybin" given in the study under discussion.
A practical approach is to take about 1.25 grams of dried cubensis, wait about an hour to see how you feel, then try another 0.5 grams if you're up for it, and then another 0.5 in half an hour. By that point you'll have ingested enough to notice effects for sure, and you'll have confidence what your personal right dose is.
Beware of tryptamine tolerance: wait 2 weeks between doses to reset your sensitivity.
Each mushroom contains a different amount of psilocybin. You can calibrate a batch by grinding them all up at once in a coffee grinder and trying a sample. (Always be careful taking powdered mushrooms that someone else has ground up, because you don't know whether anything else has been mixed in.)
Also look up "lemon tek" to prevent tummy upset.