Are all of our hairs the exact same size, calibre/thickness and colour?

This will depend and requires an in-person evaluation to answer. I suggest seeing a hair loss expert now to evaluate and go over your condition and provide you information about your best treatment options. There are great non-invasive options like prp/progesterone and/or hair transplantation. See an expert. Best, Dr. Emer

There are ethnic variations as well as individual variations in diameter, quality, texture, color, and curl of hair. As our hairs miniaturize they become thinner in diameter and lose their pigment. The differences seen could be from natural hair cycles of shedding, or hair loss is being experienced.

All hair  in the scalp has same size, calibre/thickness unless person is under the affect of AGA or TE. But color may change in different parts of the scalp with age and physical factors like sun exposure.

Hairs are normally similar in calibre but not all the exact same calibre. Individuals with telogen effluvium may experience a variation in size from new hair growth. Individuals with traction alopecia will also experience miniaturization of hairs and so will individuals with alopecia areata. Individuals with genetic hair loss also have miniaturization. But unless a person has telogen effluvium, traction, alopecia areata or genetic hair loss, the hairs are similar size. Generally, a cut off of 20 % is set as the number of follicles of reduced calibre that indicates androgenetic alopecia.    

Your hair ahould be the same caliber through out the scalp unless you are experiencing miniturixation of the hair from genetic hair loss. 

Unless you are balding or aging, your hair caliber is consistent across the entire scalp in men and women