23 Min - 26 Max for a 500 NE

It partly depends on the action being used as the barrels
have to compensate the weight of the stock / action.

I wouldn't worry about velocity - you can change the velocity
by changing powders but it's such a short range firearm it
wouldn't make a huge amount of difference.

At what point does a barrel become too short or too long?

When the gun doesn't point well or the "feel" doesn't feel right.

500 Nitro