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.
So in construction you left 20 blocks Air inside = 1Block = 1 Bucked = 1000mB .... Inputtank = 10 Buckets.... outputtank = 10 Buckets very logical for me