If you already use terrestrial reception at your house, then the antenna there will give you some idea of the size required. If you are on satellite then obviously this is not an option and you will need to get tv mob to do a "survey" i.e. signal strength/quality test and from this, they can determine which antenna is the best to use.
You could take a punt and see what neighbours are using and go the same, and if still down on signal, boost with an antenna amplifier.
I notice a lot of very large "high gain" antennas used almost everywhere here. I think they must have about 30 alignment bars......units are about 2.5m long with bracing bars underneath. These type of antennas may be to just make things uniform and easy to get a good signal where ever you are, or it may be a necessity, I do not know what signal strengths are like here. One thing for sure is you cannot get too big an antenna.
Many countries are phasing out terrestrial broadcast in favour of cable or satellite as it eliminates many of their broadcast problems and allows much more information to be transmitted, especially when systems go digital as many are and I am sure Thailand will follow.