From the i3's launch in 2014 (late-2013 in the EU) there have been two major battery upgrades. A good browse of Wikipedia will answer many questions: https://en.wikipedia.org/wiki/BMW_i3
That said, (and as a BEV owner, the below apply to battery-only i3 cars, not REx models) here are the battery capacities you'll find as you shop for a used i3. Note that in 2019, the sportier i3s was newly offered for sale.
2014 – 22kWh battery (i3 BEV)
2015 – 22kWh battery (i3 BEV)
2016 – 22kWh battery (i3 BEV)
2017 – 22kWh battery OR 33 kWh battery (i3 BEV)
2018 – 33 kWh battery (i3 BEV & i3s BEV)
2019 – 42kWh battery (i3 BEV & i3s BEV)
2020 – 42kWh battery (i3 BEV & i3s BEV)
2021 – 42kWh battery (i3 BEV & i3s BEV)
If you share the general area in which you plan to drive, those here on the forum may be able to offer further suggestions – ambient temperature and elevation changes challenge EVs more visibly than gas / Diesel vehicles.