We have to carry a wheelchair for the MIL, its a lightweight alloy one. We tried it in a Rio standing up and it was fine, tried it in a Ceed but not so good and it would not fit in a Stonic despite being a taller car. Did not buy the Rio (or any Kia this time) simply because the 3 spec only came with plastic seats and who on earth wants those.
But its surprising how many cars it did not stand up in, Kia are not unique, we bought a Fabia after much searching, excellnt for the chair and a huge discount, over £5000.
Current wheelchair stands up OK - tried in showroom. Will probably secure by tying to rear headrest if necessary.
We secure ours to the headrests with a bungy cord, works a treat but even with the handles folded its parcel shelf out.