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!
One other thing I'll mention, the package gives your directions on how to tie a loop using these pliers. Granted, we all know how to tie a d-loop, but this shows a special way using these pliers. If you use a pre-measured length of rope that is very short, you cannot tie the loop per these instructions and you will need needlenose pliers to open the loop up enough to insert the viper pliers to torque the loop tight.