Responders already mentioned it, but I will throw my $0.02 in. She is too young for topping. She looks good though. I do see some nute issues on one of the leaves. In about 3 weeks, she may be ready.
Ebay worked well for me, and I saw a sizing chart somewhere.
The cut from below is from Phresh Filter.
How do I size the proper fan to a filter?The formula for sizing a room: Length x Width x Height
Divided by 3
Gives minimum recommended fan CFM
Match fan to filter
Ex.) 10ft x 10ft x 20ft...