It’s all relative. Different people will have different ideas of what is “long” or “short”.
Some common crank lengths in millimetres are:
89, 102, 110, 114, 125, 127, 140, 150, 152, 165, 170
On the whole, longer cranks give more “torque”. That is because you have a longer lever. That means it’s easier to turn the wheel when you’re climbing a steep hill, and easier to control your speed on a long down hill.
On the whole, shorter cranks make it easier to spin the wheel fast. That is because your feet doen’t have to move in such a big circle, and your knees don’t have to go up and down so far. However, because a short crank is a short lever, hills are harder work, or more dangerous.
The length of crank isn’t the whole story. 150 mm cranks on a 24 will give you a machine that is very good on hills. On a 20, 150s will make the uni will silly to ride, and the pedals will clip the floor when you turn. On a Coker, 150s will feel about right for general use.
110s on a 20 will give you a buzzy little machine that scoots about very fast. On a Coker, they will produce a speed machine that takes some taming.
You also need to take into account your own level of ability and experience . And if you are very tall or very short, that will affect your choice.
I’d guess for trials you’d want medium-long cranks. 125 - 140?