Can this sign be used for a 'row' of chairs?
No, this sign specifically means the action of rowing a boat. If you want to talk about a row of chairs, a row of crops, or a line of people, you would use a completely different sign that usually involves flat hands indicating a straight line or arrangement.
How big should the rowing motion be?
The motion should be comfortable but large enough to clearly show the action of pulling oars. A medium-sized circular pull in front of your body is perfect. You don't need to exaggerate it too much, but it shouldn't be a tiny twitch either.
Do I need to repeat the motion?
Yes, repeating the pulling motion a couple of times is standard. This repetition helps clarify that it is an ongoing action or an activity, rather than just a single, isolated pull of the oars.