Auto Pouder has a longer grow time up to 80 days but you are right it does seem small. This is a hard question to answer because of all the variables. You did top it so being an auto it may take much longer to recuperate since they really don't have a veg state. It pretty much goes right into flowering. It looks healthy. Did you "pinch it" or top the plant more than once? The pounder grows pretty tall too. In my opinion you just let autos grow and only train branches but that is only my opinion from growing like 8 of them so someone with more auto experience might say differently. The other things that make a huge impact are the amount of sun, soil, dud plant and nutrients. So honestly I don't have a definitive answer besides topping it and recovery time. It should be showing signs of flowering by now too.