Yeah, it’s hard to tell its exact size, but I’d guess it’s between 5 to 6 inches.
Also, I wanted to ask about the venting process. Does it stress the fish out a lot? Do they continue to sulk afterward? I want to determine the sex of mine, but I’m worried about causing too much stress.
Yeah, she's probably about that size.
Mine never sulked after venting ( I kept flowerhorns in the past, lots of them, midevils and carpintis), but you can damage them while catching, damage their slime coat when holding, or they cand thrash and land on the floor - mine did.
Probably the best time is when you do a water change, as the lower water level will make it easier to catch them. Also, you can hold them using a wet towel, it's less damaging for their mucus coating.