what would be the most ideal ventilation setup for a 2 x 2.5 grow tent ?
For me ideal is no smell, little noise as possible. So i went with a 6" ac infinity fan and carbon filter for exhaust. I later added a 4" intake fan to keep a good air exchange. I usually run then at about 3/4 full speed in a 3x5 space and have to pull the door to space closed pretty hard. You might just try the 6" exhaust and filter if smell is of concern, in 2x2.5 you might be just fine with a few vents at the bottom open.
Are you going to flower in it? If not 4 in. Will work fine. 6 in. If you plan on flowering. It Also depends what light you put in, how much heat you will be dealing with. I always use the passive vents on my grow tents.
Ideally, you need to be a little more accurate - you should be trying to aim for 2-3 air changes per minute. If you are flowering as well - you will need a carbon scrubber to get the smell under control - and that adds to the pressure the fan has to work against. Be sure to use a centrifugal fan on the exhaust. Fairly small tent, probably a 4" centrif fan.
Hope that helps.
Base the ventilation on volume, not footprint.