A 5 or 6 letter ___domain name isn't necessarily significantly better than a, say, 7 or 8 letter ___domain name. twitter.com is much better than twtr.com or similar.
The trick nowadays, unless you want some unpronounceable nonsense, is to conjoin two words in some way. Let's assume your site's main topic / product / quality is "widgets." You could go with things like: gowidgets, widgetstoday, hotwidgets, redwidgets, widgetman, widgetway, widgetinside, widgetwise.. you'll find a significant amount of domains with this technique that are free to register. It does take a lot of thinking and testing to find something that's good though! You can't just join anything.
That kind tend to be more forgettable precisely because there are so many combinations of words. For example, it's been a decade or so and I still don't remember if a given office supply store is Office Max or Office Depot. But I do know if it's a Staples or not.
That's the job of branding. Microsoft, Slashdot, Burger King, Coca Cola, TechCrunch, General Motors, Morgan Stanley and Mercedes Benz aren't particularly forgettable (even though Microsoft is about as generic as it gets!)
I'm not sure there are significantly more successful single word brands than longer named brands, although it certainly seems to be true online.
The trick nowadays, unless you want some unpronounceable nonsense, is to conjoin two words in some way. Let's assume your site's main topic / product / quality is "widgets." You could go with things like: gowidgets, widgetstoday, hotwidgets, redwidgets, widgetman, widgetway, widgetinside, widgetwise.. you'll find a significant amount of domains with this technique that are free to register. It does take a lot of thinking and testing to find something that's good though! You can't just join anything.