Air admittance valve size for 14 inch diameter of soil vent pipe

I think you might be confusing diameter with circumference.

Standard soil and vent pipes are 4" So unless it's a typing error 4"

Hi, 14 inch seems very big for a soil pipe, I think you may have measured the circumference instead of the diameter (it’s an easy mistake to make), soil pipe in the uk is generally 110mm, but if it’s cast iron it may be in old money which could be 5/6 inch diameter and you’ll need a boss adaptor to get a 110mm Air Admittance Valve to fit. Thanks Andy.

As long as it's not near an opening window or ventilation grille then I wouldn't worry about it, just make sure it is terminated with a basket. If you fit an AAV externally it may be more trouble than it's worth as might freeze shut in winter.