You can also do first "onstat -d" to give you how many pages used within chunks. The page size depends on the OS (either 4K or 2K).
To verify the page size, run "onstat -b" & "buffer size" will indicate
the page size used. The used pages may not necessarily mean the number of pages used by your database, because it also includes pages used by other DB's (e.g. sysmaster, sysutils, etc.), unless the database is exclusive within the dbspace.
You can also use this sql statement to get an approximate size of the
1. run UPDATE STATISTICS to update system catalog tables.
2. SELECT SUM(rowsize * nrows) FROM systables
This is also approximate bec rowsize is the maximum rowsize for each row & does not include pages used by blobs in blobspaces.