Assuming that you runstats are current:
select t.tabschema,t.tabname,t.card/t.npages as rows_per_page, (t.card/t.npages)/ts.pagesize as avg_row_length from syscat.tables as t inner join syscat.tablespaces as ts on (t.tbspaceid = ts.tbspaceid)
This is a simple method. It does not take into account the "overhead" on each page, etc.
Andy