There are a few things to consider when buying a new Shop-Vac, and the most important one is how you'll use it. If you plan to use it mostly around the home, and intend to use it for smaller spills, or just occasionally, then a smaller model will do. These will typically have a capacity of 2 to 4 gallons, and will be quieter than the larger models but they will also be somewhat less powerful, and obviously won't have the capacity for large cleanup jobs, or jobs that must be done regularly, unless you want to have to empty the container constantly. Motor power should be around 2 to 4 horsepower (HP). But if you're looking to vacuum up after kids or pets, whether the mess be wet or dry, this may be the type of vacuum for you.
If you plan to use your shop vac more often, perhaps in a home workshop, a working garage, or on a job site as a contractor, you'll want a medium-sized Shop-Vac that can handle larger jobs without being emptied so often, and with greater suction to pick up whatever you can throw at it. These mid-sized Shop-Vacs should have between 10 and 20 gallons of storage capacity, and the motor should be around 4 to 5 HP.
If you're planning to use your vacuum for large-scale jobs, industrial cleanup or similar heavy-duty work, choose the largest class of vacuum available to you. These will hold anywhere from 15 to 25 gallons (most will be closer to 20), and should have around a 6 HP. These will pick up anything that can fit through the pipe, and will be pretty noisy, so keep some earplugs handy.