Extraccion

6" will do you good. Two 4" would work and work in/out together. Technically i bet a 4" could move enough for basic gas exchange, but you'd have to run it near 100%. If you are using carbon filters, definitely get at least a 6". 50-75% power probably be about right with one of those attached in a ~ 5x5 tent. It's always better to go a bit larger than you need so you can run them near 50% or less. They are much quieter equipment when not running 80-100%. This may not be relevant for your context if the grow area is well secluded. Regardless, get one with an EC motor... it is worth the extra price. You get 0-100% or 10-100% precise control. The shit fans might give you 75-100. The nicer EC duct fans also come with much higher quality ball-bearings than the other options, in general. TerraBloom is a manufacturer of good fans. I bregrudgingly will list AC infinity as another options, but fuck ac infinity and the shithole horse they rode in on. I'm sure there are others... EC motor is the key. They save watts and greater control. I've been running my terrabloom ones for 3-4 years at this point with no issues. I bought one of those shitty brands off amazon and returned it after the original and replacecment made loud screeeching noises within 90 days of use. Bought a terrabloom with the amazon credit and never looked back. Check prices at terrabloom and ac infinity so you can get an idea for cost... if you find an alternative that is cheaper with same attributes, go for it.