Hmm, I would say the trick is to set a destination for the bullet to hit no matter what before it's shot, then you know the start and end point of the projectile and can do the scale based on that (or, if you want it to always hit an object/around an object, just set the end point to somewhere near the target)
That way you can set the size as a % of distance from the center between both points (so it could be 50% size at start and end, and 200% at the middle).
Edit: Looks like you've got that idea started in your file already.
Double edit: Found this on my PC from an old forum example someone made and it looks like it might actually be close to what you wanted so I tweaked it a bit (original was called: Top Down Artillery.cap): https://dl.dropboxusercontent.com/u/471 ... ectile.cap
With some tweaking you could make the shadow change size too, but hopefully it's straight-forward!