It is long indeed. 7km of sand and plenty of space for you! At one end there are guesthouses and at the other there is a path for a jungle trek from the ferry.
This is the main beach of Ko Lanta. It's a few km long and has dozens of resorts and restaurants alongside but it isn't crowded! You can spot people here and there but you'll have a lot of space for yourself. Water is clear, sometimes gets wavy and the sand is soft.