The Right one for you ^
Few of us have plenty of room, and you have to face facts cages take up a lot of space. For more European and City apartments, you need to consider how it will fit into the available space. If your need a front or side entry version. Other considerations include if you care going to fold it flat and then where you are going to put it. If it is there all the time, you can also use the top of the cage as a play room table, so what height should the cage be. The bottom of the cage could have some padding in it for long term use - ideally the slave needs to be able to kneel in there without bending his head. Often the there is plenty of room, until the foam padding is put in, so included that measurement.
On the other hand he needs so be at Masters crutch heights so he can do that duty through the bars.
However when out of the cage, it is useful to have him bend over the cage as if it were a whipping horse, if the cage is too high then that becomes a problem. The slave would need to be bent over it with his feet tied to either side of the cage, and the arms the same to the top. The slaves collar can also be tied to the bars.
As a table, the slave can lie on it (need some padding from inside the cage). This them provides a nice table -- but is it long enough or is it ok for his legs to be bent over the end?
Many suppliers help you with this choice by giving you a limited choice.
For me I like to be able to sleep next to the cage some times, so I can be near my dog as he sleeps, touching him during the night should I want to, hear him wanting me.
If you have the space get one that he can sleep in easy, if you are short on space, well have bars wide enough for him to put his feet through. See how the cage can fit in your room - the cage can be a great bondage tool so you need access to all sides for best use.
One last note. You can get cages with hollow bars, this are nice and light, easily portable but to a slave, even quite inescapable, it is not like the real thing. A slave wants sublime inescapable heavy iron, quite over the top bondage, where the is no possibility of escape.
One last point, do the bolts up nice and tight, one Master I knew of have his slave undoing the bolts with his fingers and walking free to make himself some coffee... Not quite what is expected.