There are a few "sizing" factors to consider.
How far away from the transmitters are you?
Are most / all of the stations transmitting from the same tower (i.e., in Chicago, most stations are from the Sears tower or Hancock building ... makes antenna pointing easy) - otherwise you'll need something like the "flying saucer" antennas that are omni-directional, or an antenna rotator.
Where will the antenna be located in/on your building? If you are in a fairly urban area and the broadcasters are spread out, the "flying saucer" style antennas work OK. Remember for something like that, you'll need a cabling path, and the best coax you can afford (like RG/6 quad-shield, good connectors, and the tools to properly terminate it).
Be careful buying something that has an "amplifier" because the AMP is frequently at the TV end ... meaning it "amps" the noise right along with the signal ... i.e., you get nothing extra by having the amp. IF you need an amp, it should be as close to the antenna as possible (it can be powered from the TV end of the cable, as long as the cable is good enough to not have so much loss as to eat all the power before it gets to the antenna.
If you're not technical or unfamiliar with the hardware, it would be worth your while (IMO) to hire a professional installer. They can also usually come out with equipment to evaluate what you need (and then move to IWBC - "It Would Be Cool" - hardware.