Probably this is just a terminology, but 5 frames usually called a starter or nucleus colony (nuc).
Entrance reducer is important tool that can be used in many situations. Reduce entrance in cold weather, robbing etc. For a small colony a small entrance is rather necessary because it requires much less guard bees to protect colony from being robbed by other colonies. About 5 cm opening will be enough for a beginning.
The type I use at the moment is shown on the picture below:
There are several types of reducers available on the market and any particular style is rather personal choice and matter of convenience. They all work. A simple timber plank closing part of entrance works very well too. The simplest entrance reducer I’ve ever used was made of handful of dirt. I did not have anything else around but had to stop robbing. I plugged entrance with dirt and used a pencil to make a couple of holes in it. Worked quite well too