Nostril reduction during rhinoplasty procedure - how can a doctor ensure the same size on both nostrils?

You have to be very precise in measurements using fine marker and calipers up to mm accuracy. Even then there can be some asymmetry which is not perceptible to an observer.
The individual surgical technique varies depending on the experience of the surgeon.
Alarplasty is a very fine and quick procedure that can narrow the nostrils and reduce the flare to soften the appearance. It has other names as well like ‘Weir Excisions’ or ‘Nostril Reduction’. However, it is important to note that asymmetry of nostrils is common and cannot be corrected with alarplasty. Visit the alarplasty website to learn all about it.
Click on the video link below to see the details.
Regards
Dr. J

Hello and thank you for your question. While reducing the size of your nostrils is possible it's impossible to make it the same for both sides. Our faces are not symmetrical. There are some procedures such as fillers and facial implants that can minimize the difference on both sides. During rhinoplasty surgery, we match the nostrils as much as we can but bear in mind it will never be 100% equal. Best wishes.

If I do nostril reduction I often will do it with local anesthesia so that the patient can participate in the process and make sure things look as expected at the end of the procedure. It avoids a lot of blame and finger pointing.