I think my serpentine belt is a little loose and I think my tensioner is to blame. My question is how should I measure how much slack I have in the belt? Oh and it'* a brand new belt.

If you can grab the belt and move it around alot it may be bad another way to tell is start the car open the hood..and watch the belt if its bouncing around alot it is a good sighn that it may be bad..But please please never stick your hands or any body part near moving or hot parts under your cars hood

Are you sure your "brand new belt" isn't too long. Did you do a side-by-side comparison with the old one? Check a few on-line sources to see what the proper belt for your car is make sure you have the right one.

Sometimes its best to start with the obvious.