What are giants
Giants are close cousin of humans, with only a few distinguishing features: first and foremost, their size. All of them are above 3 meters tall, with some nearing 4 meters - and with a very large build: their strength is the limit of what biology would allow.
They are definitely duller than men, to get into warfare they need a human to stick with them to be sure they follow the battle plan and don't just run and smash what's the closest to them in a straight line.
As for their motoric ability, it is good, maybe not as good as a human, but as long as the object is to their scale they can operate it
We are at a point where the longbow is taken as old and slings, while not completely phased out are still on the way out. (Think end of the Hundred-year war, around that time.) But despite many attempts nobody has managed to craft a bow that would use the strength of the giants without exploding under the pressure. The only successful attempts are giant-loaded machines of siege that remove the mechanical advantage in the loading mechanism. Those contraptions are not portable even for a giant.
The actual question
Would it be possible to make a sling for giants? Are there size limitations to the projectile/ropes? And would it make for a good weapon, or would they be more useful as living battering ram?