mine is about 6 inches now, i think they get up to 7.
my tank is a 75
if it is only 2 of your fish that are hiding at the top of the tank then they are usually getting bullied. you can try to turn the lights off for a day and move all the decor in your tank around and they might come back down but if you do all that and they still stay up there then there is someone in your tank that isn't being very nice to them and you will most likely either have to get rid of them before they die or figure out who the bully is and do something with him.