I recommend to buy 4" fan and it will be fine For a 2x2 grow tent. I created a list of best cannabis growing kits and recommend 5 best fans. For get idea follow my post 1 for fan and post 2 for carbon filter and complete grow tent kits.
Also i was setting up problem dehumidifier in my grow tent. After many research and professional suggestion, solved the problem. For help other grower i publish a post on my gardening blog about dehumidifier review and how to set up with many helpful information. For check my post, Click here.