google it, but rtbs (red tail black shark) are around 5-6" max., rainbow sharks are similar, and bala sharks get to be really big, dont know an exact number, but 10+ inches i believe.
murphyk is spot on with the sizes. I have seen silver or bala sharks hit 16inches but most only get to about 12inches.
A 4ft tank is fine for rainbow or redtail sharks and for juvenile balas. Adult balas need a 6ft or bigger.
Balas are schooling fish that prefer to be in groups. Rainbow and redtails are highly territorial and are best kept on their own or in prs (1male & 1 female). Unfortunately they are very hard to sex so most people just keep one.
Redtail & rainbows will fight each other as well as their own kind.