The d loop that came on my ready to shoot bear bow is like half an inch long. I'm thinking that's a bit too long for a d loop and I have had to shorten my draw as a result. Too long
D-loop is all preference. I shoot mine about an 1/8" to a 1/4" away from the end of my nock. So, I like mine short, but I also have friends that like them longer.
It is all preference. However if it is to long It will make your anchor point further back than it should be. Here is a link to a video we just did on D loops. How to Tie D-Loop | How To Videos | Bowhunting.Com
Also the "ready to shoot" bows from big box stores are usually not set up the best. So if you don not know how to do it yourself take it in to a PRO shop. My D loops are always pretty tight to the nock, just enough room to get my release on it with out touching the nock.
I had to use a longer D loop on my first bow in order to get the draw length I needed as that bow ( my first one...) was too short for me, I have long arms... The problem I found having a 3/4'' D loop was that after the shot it would sometimes get tangled. So, in my opinion, the shorter the better. Just long enough so you can get you release on it comfortably...
It's all about anchors / draw length and preference. I prefer a slightly longer loop. Michael Braden obviously likes his a little longer as well. And this pro, Braden Gellethien