Welcome to ECF, oldgloria!
A rough calculation of vaping time is about 100mah per hour of vaping. If you want to be able to stay away from the charger long enough to go out of the house without carrying a bagful of spare batteries, then you need something in the 650mah range or higher. A 1000mah eGo batt can last 8 or ten hours easily.
We all thought we wanted something cigarette sized at first. We bought it, used it, and found that we needed more current capacity in our batteries to be able to use the really good options. Low resistance cartomizers and atomizers need a battery that can supply some power.
That's not to say that buying a small ecig is necessarily a bad idea. It gives you backup and something you can use here and there for various reasons. It's just that most of us have both and the little cig is in a box somewhere stored away. Once you use an eGo, Riva, Kgo, or one of the similar larger battery models, you will see the point of greater battery capacity.
Good luck in your search. Check out the ECF library for more info on various models, especially, "The Wonderful World of Vaping (all 6 parts)".
The ECF Library