Even 1/2" may whip at 4-5ft, depending how fast you want to spin it. But there are a lot of factors involved. If you go to 3/4", it will take a lot more power just to spin the screw. Using a higher lead screw, like 2 start 1/2-8 will let you spin it slower, but again, you may need bigger motors because it doesn't give as much mechanical advantage. You could also mount the screw under tension, which will help to minimize whipping.