Definately longer than taller. Some people will tell you that a 1 gall flower vase or tall will be fine, but ideally its not. They do like to swim, especially if you get the PK's (pure bread Malasian/Thai fighters) and not the fancy tailed versions (bread for fancy tails etc)
If all you want is one male then a 5 gall long would be ideal, if however you want to keep females then a 10 gall is minimum, as ideally you should have a min of 5 females so they can set out their pecking order.