Hello sebast,
of course the length of each cell is important.
The size of a cell depands on, what the cell is defined. If it is a numeric, e.g.: int (i think palo save an int value as a float), then palo reserved as much bits as the size of a float is defined.
If it is a text, then it is as long as the text is.
The limit of a dimension is 4 million elements (3.3 Jedox).
Just as an example:
If you have 7.000.000 lines of information and in each line, there are 2 columns (price, pieces), than you double your cube size to 16.000.000 cells.
Also when you have a dimension with 10 measures, you get 10 multiple 16.000.000 cells, and so on.
Also the structure and logic is important. If you have a "complex" tree structure for example, it will also increases your cube size.
I don't know if there is a formular and i wouldn't say/guess anything about the cube size, but if you find one, please let me know.
Regards.