You assign the database size of the whole Sybase database when you setup the server.
50% is for CbC, the rest for the other reporting.
In our huge db about 22 Gb is for CbC and 22 Gb for "normal" reporting.
In version 5 the maximum size for the database is 64 Gb, you can up-size but not downsize.
In the (web)client under the historical statistics you can determin the actual usage by selecting the amount of days and applications which you capture (include the Master_Script always). The calculation is made based upon the values you insert into the Historical Statistics settings.