i learnt my lesson the hard way...
My old sump from my old tank was too small for my 2.5ft, always wanted to change it but procastinated for goodness knows how long...
As Murphy's law would have it, when I was overseas something tripped causing my tank to overflow. The phone bill to call home to rectify it was $60, a new sump is like $80-90.
Really stupid of me...
Anyway my new sump was designed with a buffer to prevent it from overflowing during a blackout.
Best way is to overestimate what you need, or even better still calculate! then draw up a schematic, go to a tank maker and make the sump.