This "R" standard looks like a rather Singaporean thing, in other countries, the actual measurements are usually used.
No standard size to an "R", unlike "cm".
4R = 4 x 6"
5R = 5 x 7"
6R = 6 x 8"
8R = 8 x 10"
8RC = 8 x 12"
10R = 10 x 12"
12R = 12 x 16"
15R = 10 x 15"
Something like that. I might have got some wrong. The only correlation I can gather is that the "R" refers to the shorter side, except for 15R.
Regards
CK