We had to upgrade our copper tank because we emptied it before all 4 of
the
family had showered to their satisfaction.

The existing tank is hopefully a reasonable size - about 5' tall I'd guess.

Other thing to watch is the cold water tank in the loft.
It will empty faster than it can fill when the power shower is running -
you
usually take both cold and hot (cold feed to the hot tank/cistern) feeds
from this tank to provide equal pressure and avoid sucking on the mains
:-)
We have nearly emptied our loft tank in the past, before the novelty wore
off.
If it is a goodly size then you should be O.K.

I'd have to measure it but I get the feeling it's a reasonable size as well
compared to some other tanks I've seen.

Depends on how many and how long, plus mains water pressure to refill the
tank.
Like the old school test - water is flowing into the tank at x litres/sec
and flowing out at y litres/sec.......

I'll get my calulator out :-)

Thanks, Rob.