5*5*5 cube under "standart buildcraft space-time" can hold up to 2000 blocks of liquid (5*5*5*16) since pressure involved, both tanks for hot and cold coolant should be atleast 2000 blocks size.
normally player wont fill them to 100% but really large tank will make "burst" reactor viable, when player heat massive amount of coolant at once, then slowly process it.