Boys have much longer urethras than girls do. A 6Fr catheter isn't very long, and a 7 month old would probably be fine with an 8Fr since even term newborns are often big enough for an 8Fr... but that's another topic. How do you tell if you're in far enough when there's no urine? For boys, I put it in as far as I can make it go, then put a little sterile water in the balloon. If the catheter doesn't slip out when I tug gently on it, I inflate the balloon to its full capacity, usually 2-5 ml. For girls, I do almost the same thing, only because their urethras are so much shorter, I don't worry if I still have a couple of inches of catheter on the outside.