I think that the general rule of the thumb is that your baby is able to stand and turn around inside. There are some plastic ones. I have a pink one that looks just like this.
http://img.nextag.com/image/Kennel_C.../526169268.jpg
I also have some softsided sided ones. I think it's up to what you like. But I do know that the above is a must. For them to be able to turn around and stand up I mean.