Although it might still be possible to pick up some CRT televisions that are even smaller, generally when we talk about small sized televisions these days, we are looking at used 20" - 29" TVs for sale.
A useful thing to remember here is that big is not always better. For one thing, a really large screen sized TV isn't always practical - the size of the screen really does need to be appropriate for the size of the room. The closer you are going to be to the screen then the smaller the screen can be.
It's also worth noting that as our field of vision is essentially fairly limited, our eyesight can only cope with so much at any single time. Therefore, if you are going to be watching television from a distance of about a metre from the screen a metre away from the screen, anything much bigger than 32” isn’t necessary at all, in reality.
As a general rule, if you are going to be watching from less than 1.5m away from the screen then a second hand 20"-29" TV will be perfect for viewing.