I have a pair of these pliers and they do work as advertised. The shortest loop I can make with these pliers is around 1/2" ID, and that's with a piece of rope 3 5/8" long.
The pliers claim to make the loop 5/8" ID every time, so when you close the handles together, the inside of the loop will measure 5/8". But with this short of rope, you cannot fully close the handle.
Once tightened down, the loop is EXTREMELY tight, much tighter than I could ever get with needlenose pliers. I took a loop off today that I had installed with the viper pliers (had to cut it off) and the loop seperates the serving down to the string, so I'm sure your loop won't move!