HKS turbo sizes (eg. 2510, 2835, 3240, etc.) generally have the first 2 numbers as a representation of the turbine housing size and the second 2 numbers as a representation of the compressor housing size. Note, however, that these numbers are all relative within their class (eg. 25 series, 28 series, 30 series, etc.). For example, the cold side setup on a 2540 turbo is not equivalent to the cold side setup on a 3040 turbo despite the second 2 numbers being the same.
If you want all those sizing details you'll need to get your hands on the "HKS Goods Master". It has all the details you'd need. I'm not sure if anyone on this board has the document. If they do maybe they can scan the relevant pages? You could also try the people on the the following boards as a lot of them tend to use these Japanese turbos: