For IM, all the medical research indicates that glutes, thighs/quads are the two best places to inject, especially larger volumes, but delts are also very good for smaller volumes. The larger the muscle, the longer the needle. It can range from a recommended 1.5-3 inches!! For a larger person, a 2+ inch needle is not uncommon.
As weight lifters, we tend to stay away from needles larger than 1.5 because they look scary. xD
Always try to use the smallest gauge needle possible to decrease scar tissue though. I use 23g because some oils I use are very thick. I like the high mg/ml stuff for less injections.