The 650mah eGo-T is shorter in length than the 1000mah due to battery length but overall they are the same diameter.
Heres a pic of my 1000mah eGo-T next a dollar bill to show approx size. Once charged, it should last you pretty much all day. Also with the tank system, a full tank should last a day as well.
Personally I would reccomend the 650maH to start with. If you feel you need more battery life you can always just buy 1000mah batteries and you'll be good to go.
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