What size shop vac do I actually need?

For occasional car cleanups and small spills, a 2.5- to 4-gallon unit is enough and far easier to store. For workshop, garage, or renovation work, 5 to 6 gallons is the sweet spot, and 12 to 16 gallons is appropriate if you're routinely doing drywall, flooring, or wet cleanup jobs where you don't want to keep emptying the tank.