Yes its definatley a Vespicula depressifrons or wasp fish if you want a common name. Its correct these fish are venemous so you have to be very carefull when going in the tank to do maintanace but it wont kill you dont worry. As for size in the wild 4" is about right but more like 2.5" in home aquaria. I wouldnt suggest keeping this fish with any tank mates really as it is a predator and will take down smaller fish and will give a nasty sting to any other fish especially bottom dwellers. They can be quiet fussy to get on to dead foods aswell. and spend most of there time on the bottom.