When playing with daggers that do 1d4 of damage I've found the time will come in a game that someone wields a larger or smaller blade and will ask me...what's the damage rating for this?
The easy answer is 1d4 (for dagger like weapons) and while simple and easy it just isn't always satisfying in the long run.
Of course there isn't typically a lot of wiggle room. The die type above 1d4 is 1d6 and the 1d3 and 1d5 are both easy to fake with other dice but have limitations. One factor with daggers and other short blades that is troublesome is the damage bonus for high strength. If a knife does 1d3 and a dirk does 1d4+1 a character with a +2 bonus to damage is going to mitigate those differences quickly. One solution (and the one used in this post) is to stretch out the low end of damage roll.
Stretch out the low end by using a die and subtracting a number so as to extend how often a 1 is generated b a roll ( this all works off the practice of allowing no score to be less then 1 on a damage roll regardless of math).
Let's say we want a dagger doing a solid 1d4 of damage as the baseline for small blade damage ratings. We could well do otherwise but the 1d4 is familiar and established in a lot of games and in players minds.
How about a knife pretty much the same size as a dagger that isn't really as well designed for combat. Such a knife can certainly do just as much damage as a dagger just not as often. So let's move up a die type and apply a subtraction so the max damage for a knife doesn't exceed that of a dagger. I'm picking 1d6 so applying a 2 point modifier to that gives us a damage roll of 1d6-2 for a knife. Such a knife could do as much damage as a dagger but is more likely to do less damage. the possible damage rolls for 1d6-2 are 1,1,1,2,3 and 4 which give us an average damage of 2 as opposed to the average damage roll of a dagger which is 2.5.
Now let's move to a weapon a bit heftier then a dagger a big solid knife of which there are many varieties in the real world many of them fighting knives that are more substantial then a dagger. I'm going to used the invented identification of Broad Knife for such a weapon. Well use a higher damage range but we'll apply a modifier that allows for damage to be higher then a dagger on occasion. I'm going with 2d4 -3 here, this gives us an average damage of 2.25 with the occasional chance to do more damage then a dagger but keeps us out of a short swords damage range. The dagger is still a superior overall weapon vs the broad knife but for those that want a bowie knife the differentiation adds a little to the game and makes the weapons fee different in play.
Collected Table of Daggers and Other Blades
*note averages are based range of multiple scores of dice not simply the finally generated scores.
When applying a damage bonus or penalty factor it into the roll do not apply it after the roll so a +2 damage bonus due to strength would yield a damage range of 2d4-1 with a Broad Knife (letting us emulate the doughty Saxon warrior with his scarmisax). A Misericorde in the hands of a knight with a +2 damage bonus would be a weapon that did 1d10-2 damage. Applying the strength bonus after the roll would give us a different range of damage (applying +2 after one resolved 2d4-3 would give us a damage range of 3 to 7 as opposed to 1 to 7). Add magical bonuses to the damage roll after penalties and bonuses for strength are accessed (so by example a +1 Misericorde would do 2-7 pts of damage as opposed to 1-7 pts of damage).
This principal could be extended to other weapons as well but I'll leave it here for now for consideration and experimentation.