How many mm of nasal nostril could be reduced from my nose? (Photo)

The best way to determine what specific changes will look good on your face, and what changes are surgically possible, is to do your research to find the top rhinoplasty specialists and consult with several of them in person.  During your consultation, your photos can be computer-morphed to show how the changes to your nose will affect your appearance.  A rhinoplasty specialist will have the experience to determine whether the morphed images are surgically attainable. Good luck!

Hello,  Thank you for provided photo, and information.Each has unique proportion, according to that proportion a new desired look can be achieved by Rhinoplasty that executed well, an alar excisin procedure to make smaller your nostrils can be performed.By Closed and Atraumatic technique you will be provided maximum healing, mininum tissue damage without a scar in your columella.Best Regards,

Thank you so much for submitting your question. 
The ideal alar width is approximately the same as intercanthal (width of your eye) width. Looking at your photos I would agree you do have widening of your alar region. TO narrow it will likely require external incisions along the exterior of the nose called alar base resection. The degree to which you can narrow this is based on being able to maintain the external valve of the nose for breathing purposes. A surgeon in person could review this more with you. 
Best of luck and I hope this answers your questions. 

When I read your post I didn't believe it but looking at the photo it does look 6cm wide at the alar base. The desired size is usually equal to the distance between the inner and outer corners of each eye (the intercanthal distance normally is 3.2cm). That would require an alar base reduction of 1.4cm from each nostril rim. That is more than usual so I suspect you will also need some thinning of the nostril rims in order to decrease the resection size to avoid a pinched nostril appearance.

Based upon your photo, you could probably be able to achieve a result where nostril width would be reduced to approximately half the size of your present nostril width through Rhinoplasty with Nostril Narrowing.  I would suggest a formal in person evaluation with a facial plastic surgeon who is an ethnic rhinoplasty specialist with extensive experience in nostril narrowing surgery.  Make sure you see numerous before and after photos with results you like and seek.  Request computer imaging.  This step ensures that you and your surgeon are united as to the desired end result as well as making sure the end result can realistically be achieved.Best wishes,Dr. S