How can I tell how much space an index is using in a specfic tablespace ?

If you run runstats on the table with "and indexes all" option, you can then query the view syscat.indexes to see various statistics. The NLEAF will tell you how many pages there are (multiply by tablspaces pagesize to get total size). There are also some non-leaf pages, but NLEAF will usually get you with a few percent of the number.