Cooling duct are used to precisely cool down plastic right after it was lay down without cooling down build plate (to avoid warping issue), this enable you to print bigger overhang without the need of support which end up in greater details.
Unless you have knowledge to create CFD trace, I would avoid it, they were/are tons of poor designs like ring duct which are very ineffective/unprecise. You don't need really high pressure but blower fan are better than radial fan for almost all designs is why you see them the most (they are also more than pressure which are to take into account but don't recall).
As of reducing print bed, it doesn't, But if you wish to full the plate with parts and use the "print one object at a time" setting, it will reduce the number of object you can put on a plate, otherwise it will knock down parts.
As an exemple, my first printer which was the OG ender 3 with petsfang duct get better overhangs than my bambulab A1 with stock duct.
The most popular way (and might also be the best) is to use z2m instead of deconz