I have babies, my male is about an inch (he is a plakat) and my female is so tiny still. You have a youngster, too! C:
It greatly depends... heck, king bettas at 3 months old I have seen to be over two inches+! I wouldn't get a king, I feel like they need at least 14 gallons because they're just so big!
When you have the money, consider a 10 gallon set up. Bettas don't out grow that! It isn't hard to find combos (tank, hooded light, filter) for around $30-$35! That's about the same for a smaller set up and LESS than 5 gallons!
He is so gorgeous!