If you think you might want a Subwoofer along with the soundbar, that might limit your placement options a bit, but you'll likely need less space than you'd expect. This is because almost all of these subwoofers are wireless. That is, they receive the audio signal wirelessly through the soundbar, so you don't have to connect any cables between the soundbar and the subwoofer. However, you will need to plug your subwoofer into an outlet for energy.
Also, it is difficult for our ears to tell where the low frequencies produced by the subs are coming from. Bass frequencies have wavelengths in meters and spread throughout the room. So you can usually position the subwoofer pretty much anywhere in the room and have it mix well sonically with the sounds your soundbar produces - you can conveniently place your subwoofer next to your sofa or in a corner of the room.