For an unobstructed view use convex mirrors or dome mirrors. Convex mirrors provide vision around corners, blind spots and aisle ways. Ideal for doorways to see who is coming and going.
So which size convex mirror should your order?
30-45 cm - 0-5 metres
45-60cm - 5-10 metres
60-76cm - 10-15 metres
76-90cm - 15-30 metres
Measure the distance from the person to the mirror, then from the mirror to the object to work out the distance so you know what size convex mirror is needed.