There is no evidence showing that big TV is more harmful to eyes than small TV. The disadvantage of watching TV depends on how long you are sitting in front of TV and the distance between you and the TV. So, as the point of protecting eyes, it's the same for big TV and small TV. While, for protect eyes, you'd better sit farther in front of big TV than small TV.
Personally, if you want to protect your eyes, you shall keep yourself away from TV screen and computer screen. But it seems impossible. If you plan to buy TV set, i suggest big TV set. Because the bigger TV screen can make you easy to capture the image thus to reduce eye strain that like small screen give you. And you can adjust your TV screen, usually, a mild point on brightness and backlighting is best. And take a regular break when you see TV that can help you keep health eyes.
Actually whatever the small tv or big tv will be bad for your eyes because of the radiation from the screen. You should control the time of your using of computers or tvs. In addition, you could also wear the computer goggles to protect your eyes from the radiation. If you feel tired at the eyes, you need to stop and have the rest for the eyes.