The i3 uses a beam style wiper blade, with the OE part being manufactured by Bosch in Belgium. It appears to be based off a European market wiper blade, the AP750U. This would be a 29.5 inch blade here in the U.S.A. Many auto parts stores stock Bosch ICON wipers, which are very high quality, and quite comparable to the OE wipers. Unfortunately, this is where the fun begins. Bosch ICON wipers are available in up to 28 inch sizes. Most aftermarket parts catalogs list the i3 wipers as being 28-inch Driver, 21-inch Passenger (This is U.S.A. LHD spec). The passenger side wiper is indeed 21 inch on the OE part, but as I mentioned earlier, the stock driver wiper blade is about 29.5 inches. There are other types of beam style wipers that can be fitted (please, don't use the non beam style wipers with your i3), and I also tested the Trico Ultra which fits and wipes well too.
The problem is, the 28 inch wipers are just barely NOT long enough to wipe in front of the forward facing camera. The stock 750mm wipers do clear that area. If your i3 does not have the Technology Package (Adaptive Cruise Control), then don't worry about that, as the 28-inch wipers clear a plenty big enough sight area, and also cover the auto wiper sensor.
TL;DR, if your i3 doesn't have the technology package, you shouldn't have any troubles with the commonly available 28 inch wipers. If it does have the technology package, you'll need to get the OEM wipers that are long enough (Or, if you live somewhere where you can get Bosch AP750U wipers easy enough), else you may get a "camera obstructed" check control message, and your cruise control features will stop working.