Do I need to match the vent size to my duct opening?

Yes, and it is the single most common source of returns in verified-purchase reviews. Booster fans and smart vents are sold by exact duct cutout dimensions (4x10, 6x10, 6x12, 10x10, 6x14, and so on). Measure the actual hole in your floor or wall, not the decorative faceplate, before buying, since these products only fit one size.