I've seen one.
Was back in the 70's, diving in the Atlantic in Panana. Was during the dry season..big waves, clear water in the Atlantic then.
Location was an off shore island and the rocks around it.
It was common to see single large barracuda there (perhaps the same one), and they seemed to be very territorial...either following you, or getting in your face with gill flaring.
This one was around 5 ft, and seemed very agitated we were diving there...it followed Phil very closely....the whole dive.
Then, when he found a lobster, and had his arms under a ledge in about 45 ft, it came up behind him...did some strange motions for about 30 seconds (I was about 50 ft away at the time, and when I saw it, headed to them)..and then struck, at the same time Phil was ducting down to see under the ledge... it hit the regulator at full speed, knocking Phil into the ledge, and knocking pieces of it's teeth out.
It is possible it was striking at his regulator, don't know, but it left Phil with some scrapes on his forehead and one dazed barricuda. However, had Phil not ducked and moved forward, he would have been hit in the lower back of the head, and not the valve.
Size... I consider 5 ft the normal "really big" barricuda. Have seen bigger, but nothing over 7 ft. Not that I profess to have seen every one, but given the size distribution I have seen over the years, would expect it to be something around that 7 ft (although the largest I have seen was perhaps 6 - 6.5 ft.