RE: LiPo battery size => flight duration?
If your just looking for additional flight duration like the thread title says, don't increase the voltage. Use a 3s battery larger than 1300mAh, such as a 3s 1500mAh or a 3s 1800mAh lipo. Heck even a 2100mAh 3s lipo if your plane can handle the extra weight.