Counter does not reflect the base power, it reflects the total damage done. So if you're WObbuffet takes 666 damage from a physical attack and it uses counter, its attacker will take 1332 damage and probably die. According to this, counter isn't affected by type effectiveness
Although Counter deals Fighting-type damage, it is not affected by type effectiveness (so it can affect Ghost-type Pokémon)
However it was changed in gen two
Furthermore, Ghost-type Pokémon are now immune to Counter
This thread also explains how counter works and while that thread is for gen four which is god knows how long ago, nothing concerning the main mechanics surrounding counter has changed