The standard TV tray table is around 23 inches wide x 17 inches long x 26 inches high, but that's by no means the only set of dimensions you're limited to. If you will be buying a unit with an adjustable height feature, then, of course, you won’t have to do any measuring. However, when the unit is fixed and can’t be extended, you need to know how low or high a tray table will be in relation to your sofa or chair. If you don’t get the right height, you’ll end up not using the tray table which makes it useless.
The right size for your tray table is one that is not too big that it takes up too much floor space. It is also not too small that you’ll need to squeeze items on top of the table. For more flexibility, choose a table that collapses to a small size and extends. It will also allow for easy storage.