I "think" it is not 15R, and is 12R. 4x6 is 4R not 6R. If you have 3R or 4R photos on hand...then you can measure. 8x10 and 8x12 are both 8R, not 10R or 12R.
I'm not sure. If I'm right, on the RGB price list, there is no such thing as 15R. If you want a print of that size, I think you'll just have to specify.
I don't know if '15R' or '12R'exist. If they do, the dimensions should be for 15R should be 15" X 17". Similarly for '12R' the dimensions should be 12" X 14".