[3.5] '%' on hit effect on a weapon.

[SneeR]

Just be aware that the less damage a weapon does, the more often its max roll comes up, which will produce the effect more often.
Let me show you for effect how often max damage is acheived:

2d6: 1/6 x 1/6 =1/36=2.7% of hits
2d4: 1/4 x 1/4 =1/16=6.3% of hits
1d10: 1/10 =10% of hits
1d8: 1/8 =12.5% of hits
1d6: 1/6 =16.7% of hits
1d4: 1/4 =25% of hits

[ariasderros]

And in that vein, don't think you'll get away with giving him a greatsword, and it staying that 2.7%, because if the effect is worth it, he'll find one of the many ways of shrinking.