There are a few issues that need to be faced when upsizing bows
Firstly, we consider why the bow is so effective (or was, back in the day). This is more or less easily answered: the arrows were downright sharp. Whether it was to pierce armour (which it usually wasn't) or take down a few horses (hehe, Agincourt), the bow and arrow was effective because sharp things are annoying.
Secondly, we consider how this affects our giants using bows. Ok, so somehow, we manage to construct a really-long long bow™ (RLLB). That means we need pretty massive arrows, right? I guarantee firing a correctly scaled arrow from our RLLB will be like launching a battering ram - it will still be devastating but we might as well set it on fire, make it bigger, make it faster, make it stronger, scrap the useless tip that isn't really a tip any more, and overall, make people fear it. Then you call it the fiery ram from hell™ (FRFH).
Thirdly, we consider how our FRFH will be launched by a mere RLLB. Frankly, it won't. It would be like launching a land-to-air missile with a spoon. So we need a new launch method, but still a bow. The RLLB gets an upgrade! Of course, we make it out of steel, because that makes it shiny and shiny things are better. We still have some pretty out of date string here, so we'll change that to... more pieces of string (a few hundred should be good) that rest parallel to the original. Because more stuff means more power, right?
Finally, we consider how this thing will work. Pretty much, these medieval missiles work best used in a siege, but hitting a warrior with a FRFH? More reliable than a measly arrow.
To sum up: We can't have little bows for big giants, and we can't have little arrows for big bows. And having a sharp tip on a large log doesn't do much, because tips are for piercing, and you threw piercing out the window the moment you said giant. So this new weapon is for crushing everything, but is probably ideal for destroying castle walls. And hitting anything that will make a big sound when it gets hit.
: Being crushed is probably more annoying
: Fire makes it go faster <<<< Joke <<<<
: Couldn't think of anything better than more