The type and style of speed bag you should hang depends on two factors. First, how new you are to boxing and working a speed bag. Second, what you’re wanting to accomplish.
If you're just beginning, you may want to start with a larger bag. As you progress and become more skilled at it, you’ll want to graduate to a smaller and more difficult target. The smaller the bag, the faster it moves. A smaller bag is obviously more difficult to hit and harder to control. A smaller bag helps develop greater hand speed and demands greater focus. Both are beneficial to you in the ring.
If, however, you want to have a more substantial target, expend more effort and develop stronger shoulders, you should choose a larger bag. The bigger bag requires more force to hit and keep it moving. It helps enhance hand-eye coordination and speed just like a smaller bag, just not to quite the same degree.
You should also consider experiencing the best of both worlds by switching between several different size speed bags. The variety and changing demands is good for mixing it up and making you more adaptable. It forces you to change-up the amount of force you have to use to keep the bag moving and keep improving. Keeping your mind and body guessing and forcing it to constantly think, will better prepare you for the unpredictability you may face in the boxing ring.