Pendulum Project PLEASE HELP ME?

ok so my science project is about changing the weights, string lengths, and arc sizes. I have everything set to go but for my experiment what size strings should i use? i was planning 4 inches, 8 inches, and 12 inches, but i was wondering if i need like 3 foot, 5 foot and 8 foot to really make a difference? what do i use feet or inches? thanks a lot :)