SQL Server 2005: Full-text search space requirements
Here is some old code I wrote for SQL 2000.  It still works on SQL 2005.  You can use this to come up with a rough estimate of how much space you are using per document, then multiply that by your estimated number of documents.

   select 'CatalogName'   = left([name],30),
          'Status'        = case(FULLTEXTCATALOGPROPERTY ([name],'populatestatus'))
                              when 0 then 'Idle'
                              when 1 then 'Full population in progress'
                              when 2 then 'Paused'
                              when 3 then 'Throttled'
                              when 4 then 'Recovering'
                              when 5 then 'Shutdown'
                              when 6 then 'Incremental population in progress'
                              when 7 then 'Building index'
                              when 8 then 'Disk is full.  Paused.'
                              when 9 then 'Change tracking'
                              else 'Unknown'       
                            end,
          'ItemCount'     = FULLTEXTCATALOGPROPERTY (name,'itemcount'),
          'IndexSize(MB)' = FULLTEXTCATALOGPROPERTY (name,'indexsize'),
          'UniqueWords'   = FULLTEXTCATALOGPROPERTY (name,'uniquekeycount'),
          'ErrorLogBytes' = FULLTEXTCATALOGPROPERTY (name,'logsize'),
          'Location'      = left(s.path,50) 
   from sysfulltextcatalogs s