You can verify the sex by looking for spurs. If you find them, it is a male. As for size, that doesn't look out of line. I have adult PROVEN females that are 4' long that are much older than 2 years old. On the flipside, I have males and females in the 8' range. Size is fairly variable, so long as he's healthy and feeding, I wouldn't be concerned by that animals size at all. Congrats on the purchase, they're excellent boas!