For a 25lb dog you would be looking at a dosage of around 25mg of Benadryl. The 1mg per pound is a general rule that most veterinary sites I have come across have recommended, however you may want to double check with a trained vet just to be sure.
It's also important to only treat your dog with Benadryl when it is necessary.
Benadryl is an antihistamine which can be used to treat the following ailments in dogs:
Insect and flea bites and stings
Side-effect of vaccinations
Muscle tremors and knots
It can even be used to treat anxiety, for long car journeys or fireworks night.
The dosage is pretty straightforward, based predominantly on weight of the dog. Check out the following chart:
Bendaryl is an over the counter medication and therefore is completely safe, providing you follow the recommended dosage.