I have seen people keep them in pens smaller than ours, it just depends on each individual setup. The most important part is that they can get away from things that would scare them- like stray dogs trying to get into the pen.
Our pens are about 50 ' by 200' with ajoining pens on 3 sides. They have a small shed (10 x 12) in the center, which they can access to get out of bad weather. The front of the shed has fiber glass panels on the front to let in light and sun during the winter. When temps are below zero we sometimes turn on a heat lamp in the shed for them.
We feed a custom mix- which is close to a 14% sweet feed & alfalfa/grass hay, along with apples, carrots & other fruit and veggies.
Dik dik are an antelope. Muntjac are a deer.