Photo and Paper Size? It's confusing.

Doubting on the photo paper size, so i do a check out on www. And This is what i found on the net today,

A size chart illustrating the ISO A series and a comparison with American letter and legal formats.

From the picture illustration, now i understand that each size of the paper is the double size of it's Smaller paper before.

Example:

A3 = 2x A4

A2 = 2x A3

A1 = 2x A2

The ISO paper sizes

So, from the explanation given above we can guess how big the paper would be. The best comparison is by using the A4 size paper, so to know the A3 size you just need to double up the A4 paper.

In order to ask the shop to print a photo, is it possible to ask them in the 'A' term? Or from what i know mostly the store will provide the photo pictures in 'R' therm such 5R, 4R, 8R etc.

Photograph sizes

By comparing the photograph and paper size above, it does confuse right. There's no such 4R = A6 or 8R = A4 in actual size? Hm.. Need to find out more regarding this paper and photo size. 

I get this source from wikipedia, check it out: 

http://en.wikipedia.org/wiki/Paper_size

Me:
Sorry for my poor english