We have had a few Giantism characters in our game and our house rule is that all weapons, armour etc is the maximum weight according to HARP.
For instance, a Mace 2-4lbs would be:
2lbs small characters (eg Halflings)
3lbs medium characters (eg Dwarves, Humans, Gryx)
4lbs giant creatures (Giantism)
We also allow Giantism characters to be able to use items found (such as rings, cloaks, pendants etc) that, in theory, would be too small to fit around fingers, necks etc
If a Giantism character in our game wanted to be able to do extra damage because of size (for instance an oversize shuriken as above) I would only allow it if the player took significant reductions in DB since it would be a lot easier to hit a 10' 700lbs player than a 5' 8'' human. I would then allow the player to make the decision as to what they prefer, being able to deliver more damage and receiving more damage or playing as a standard HARP character.
Hope this helps.....