Because all i3's have the same maximum DC fast charging rate of 50 kW (usually ~45 kW in real world conditions), how much the battery cells are stressed depends on the battery pack size. As a result, DC fast charging is twice as stressful for a 60 Ah battery pack as for a 120 Ah battery pack.
The charge or discharge rate can be expressed as a function of the battery pack's total capacity. A 45 kW charge rate for a 22 kWh (60 Ah) battery pack would be ~2C but would be ~1C for a 120 Ah battery pack. I think that a 1C charging rate for a Li-ion battery cell isn't considered stressful. 2C is certainly more stressful, but it doesn't occur over the entire charge level range. Maybe others will provide more information.
Because 60 Ah battery packs seem much more susceptible to degradation than 94 Ah or 120 Ah packs, I limit DC fast charging for our 60 Ah battery pack. However, I would not worry about DC fast charging a 94 Ah or 120 Ah battery pack.
2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE