You should check out ExpertFlyer.com, which will tell you how many seats are left for sale on a given flight and let you view the seat map (which is not a reliable means of determining how full a flight is, but will tell you where the empty seats currently are).

It's a rare long haul these days that is empty enough to let you spread out across 3+ seats. Airlines need to fill all the seats to make money. Unless you're booking the day of your flight, finding an empty row in coach will boil down to luck. If you want to stretch out, get some miles under your belt and upgrade, or buy a business class ticket outright.