Antennagate: how much of the phone's side should I not touch?
Most cases are cheap and will resolve this issue for you completely (and if you have enough money for an iPhone, then you have enough money for a case). I got one and the problem was solved.

UPDATE: Based on the comment, if a piece a tape really is necessary, then by the looks of this product (currently sold out) , you are looking at roughly 1 in/2.5 cm in length starting at the mic/speaker hole on the bottom-left. Good luck!