As a short nage myself: open enough for keeping balance but close enough to avoid uke falling on my knee.
This is my experience as well.
When done slowly, a wider stance does make it easier to keep your balance. When done fast, however, the risk of uke falling on the knee seems to me to be much more of a concern.
If you can't keep your balance with a stance narrow enough to be safe, the solution is probably to practice until you can, not to adjust your stance to compensate.