I would imagine most people would advise a six foot tank. they shouldnt be kept in pairs as quite often one will bully the other but groups are normally ok. I would not keep one unless i had an 8 ft tank as they grow big and are suprisingly actie. the smallest species is Datnioides polota - Silver Tiger Fish which only reaches 12 inches however this particular species appreciates a brackish tank.
Any thing that cant be considered food. dats have big mouths so you should be thinking larger tank mates like large silverdollars or larger cichlids but they cant be to boisterous as you wont have a happy dat. a nice roup of sevs or geos would be nice...........You should start your own thread you would et more answers that way