It is true that DLR is much smaller and more compact than WDW. It's also true that there is much less to do. If you go to WDW to golf and enjoy water sports, and if Epcot is your favorite park, than you will likely be disappointed in DL. If you expect the DLR to be very similar to WDW, then you may be disappointed. But most people who love WDW also love DL; it's just different. I love WDW, but I have no trouble spending 6 days at DL (I did last August, and ended up converting my PH to APs because we were ready for more). Many people feel 3 or 4 days is enough, but between DTD, DCA, the resort hotels and, of course, the original DL, I think you will be able to enjoy six days, especially if you are staying at the DLH or GCH.
(But if you do start to get bored, you can rent a car and take a day trip to San Diego, Hollywood, the beach, or any number of other places).