I was just in the same boat and the math I saw was something like this:
You have a 2x2x5 foot tent (roughly) which is 20 cubic feet, so, in order to change the air once every minute, you need 20 cfm, to change it twice you'll need 40 cfm. A filter will cost you about 50% efficiency so you'll need 30 - 60 CFM. Any "turns" in the ducting will also cost efficiency, but, as most 4" fans start around the area of 200, which is going to turn the tent inside-out at full power, you'll want a 4" you can adjust down as much as possible.
I am using a 3x3x8 tent so my CFM needs are 72 CFM for one cycle per minute, plus 50% for a fan, which is roughly 100.
The ACInfinity with digital adjustment is all the rage right now -myself, I opted for the Mars Hydro fan that appears to be an attempt to copy it, because it was about $30 less expensive. Listing claims it has 10 speed settings which means you would be able to turn it down low enough that it won't suck the walls of your tent inward but should still allow for good circulation..
The cheaper version of this has a manual dial that only has 3 settings - I went with the digital one because 1) 10 settings give me more flexibility, and 2) it includes a themometer/humidity probe
Amazon.com : MARS HYDRO 4" Inline Duct Fan 4 Inch Ventilation Exhaust Fan with Temperature Humidity Controller for Heating Cooling Booster, Grow Tents, Hydroponics(4" with Thermostat Control) : Garden & Outdoor
In theory, if this fan has 10 speeds, with each speed level representing 20 CFM, a setting of 2 would be 40 cfm and 3 would be 60 cfm, which is right in your wheelhouse...and if you need more you can just turn it up. I figure mine will be a 5 or 6.