I am not in to the gallon thing... so is a imperial or US gallons you measuring with ?
My local Doha one is the size of you future sump... but lack of time has proven that it was the right joyce to keep it low profile.
A new one with approx 1500liter in in planning for my living room, but all the commercial ones have priority since they are paying projects :-).

How much space you have for your sump ? Its a great way to increase overall volume
to make a as large as possible sump... Plus I always include fresh water storage in to it
as well, so that the evaporated water can be topped off automatically.

Marc