They don't need a large tank but a heater is recommended. You do not have to have a filter - if you don't you will need to change the water every 1-2 days, if you do have a filter one with a low current is better as males find it too hard to swim in a lot of current.
In my opinion at least 20 litres for a single fish with a small heater and filter (sponge filters run by an airpump work really well), temperature 22-27 degrees (mine are kept around 24 degrees), a few plants either artifical or real.
Have a read if the following site www.bcbetta.com or look down below in the anabantoids section for more info on keeping these.
They are lovely fish.