Sizing your tank depends a lot on what you want to do with it, and your safety level tolerance.
For example, is your flow the same all year long? What about in the middle of winter, or at the end of a dry spell in the summer?
Do you want to have any provisions for fire fighting? Having a few hundred extra gallons around might mean the difference between putting a smaller fire out, and having the home burn down. How far are you from the nearest fire station? How long would it take for them to get there, and where is the nearest place they could fill a pumper (if they even have one?!)? If you're using a spring, you may be out in the country, so there may not be a nearby fire hydrant, so any fire-fighting water would have to come from either your storage, or the pumper truck...neither one is a bottomless source.
You might get by through a weekend in a trailer with a 20-30g water tank, but it could easily be all used up by one person on one shower. What do you want at your place?