By the look on the picture, it's blade length is far too long for a typical dagger and more closely resembles, what i recall as, a leaf bladed short sword which were common during the european bronze age.
From what i recall there were actualy knives in european and asian history which were the length of short or regular swords, but mostly because of the role as cutting weapon. Daggers being defined as one handed thrusting weapons for brawling distance tended to remain quite short.
In addition to that, it is drawn with a hilt suited to be wielded with both hands, which makes it rather problematic to use in typical knife/dagger fashion since the chance of the hand slipping up and down during thrusts is increased.
Now of course such details are often of no importance for many artist and they just draw what they or the writer thinks looks cool, suitable or menancing. So the design might have been of no importance and the artist really just wanted to give Storm a long dagger with two handed hilt.
BUT, considering that in other pictures Storm was drawn with a lightning bladed long sword, i hazard the guess this weapon is designed with a bronze age short sword blade to make it look ancient and has a two handed hilt because she turns it into a longsword by infusing it with lighting.
Infusing a shorter blade with something to increase it's blade length is not uncommon in fiction after all.