Struggling with Japanese Katakana

When working with non-Western characters you sometimes encounter special problems. For instance the Katakana syllabary, a set of 47 different sounds from Japanese that are used to represent Western names and objects. The basic sounds are not enough to get all the different sounds in languages other than Japanese. Therefore it is possible to combina two characters to create a new one. When doing this, the first character is standard in height, the second is smaller. An example may be seen in the chart I scanned. Take the "nyo", which is made from the character "ni", followed bij a smaller "yo". The result is pronounced as "nyo".

The problem I now have is: how much smaller should the second character be in relation to the first? It looks like 50 percent but it is hard to tell. It could be dependant on the style of writing (whether the text is indeed written or printed) or the person writing. I guess I'll have to do some experimenting, unless someone has a bright idea.