There is practically no limit. You can only have 13 customer defined dimensions. You need to have similar characteristics assigned to one dimension. If you assign unrelated ones, you increase the cardinality and you end up getting erroneous records in the cube. So, your limitation really is to have 13 related groups of characteristics.
Now what I know about that is max 255 chars and 255 key figures can be stored. Here I am talking about the individual planning levels (like product, product group, customer, location and so on) and not the "DATA". I know data storage can not be defined in numbers and no significance too.
I hope my question is clearer now !!