My wife and I test drove an i3 at the weekend, and one criteria was that our daughters car seat fitted. It is a Cybex Sirona - http://cybex-online.com/gb/carseats/sirona.html
, and is quite a sizeable seat which rotates to allow for ease of child installation. It fitted fine behind the drivers seat, and with the seat in position for me (5'9" - 175cm) it fitted well. We normally have the child seat behind the passenger to allow easy access from the kerb, so space should not be an issue.
On another note, we also have a Bugaboo Cameleon 3 pushchair - https://www.bugaboo.com/GB/en_GB/stroll ... -cameleon3
(again quite large), and that fitted in the boot. There was no other space in the boot though.
We have a 24 hours test drive this coming weekend, where decisions will be made...
Hope this helps