Do you need the size of an existing catalog? NicC's response is good, though there are other ways. The size values for used space in a catalog are not reliable.
Do you need a method to estimate the size of a new catalog? I don't know any way to do this since it depends on the number of entries and the average size of an entry. The number of entries can't be predicted in advance. The size of an entry is an unknown quantity since IBM refuses to publicize the structure of catalog entries. Worse, the size of an entry is variable and very hard to predict in advance, even if the exact structure of the entries were known. Even if you knew the exact size of an entry, you run into the issue of number of entries, plus the added complication of the distribution of type of entries.