Unless you have an extremely large budget, you will probably not want to fill your living room with playpen balls, as this will be quite costly. A small spare room or, as we’ve done, a balcony/patio area can be great because you can partition off part of the area for the ball pit, allowing you to keep the size (and therefore cost) to a minimum. If you’re choosing to use your balcony, be sure that it has a roof or that you will otherwise be able to seal the area so no balls to escape and no water or outside dangers (such as bugs) can get in. Also remember that the position of you ball pit is important for both safety and economical reasons. Building your ball pit in a corner, for instance, can eliminate the need for you to build two sides of your frame.
Depending on the number of people you want to fit into it, partitioning a 4′x8′ section will give you plenty of space for 3-4 people to play in, or 1-2 people to really jump around in. 3′ tall should be enough for the average person to climb in and out of. You should also consider how deep you want your balls to be. We felt about 2.5 ft deep would be enough to immerse a seated person while still not feeling like you’re drowning in balls, or without them spilling over. This calculator
, which has been very accurate in my experience, will help you estimate the amount of balls you will need and the approximate amount you should expect to pay for them.