Serendipity did a really good youtube video on making your own playpen, it dismantled really easily so you could store it under your bed or in the wardrobe without much difficulty, you could put an old towel on the floor underneath it to protect the carpet too. With something like this you could make it as big or as small as you can fit.
Serendipity's Video (hope you don't mind!)
https://www.youtube.com/watch?v=AvXJ-Jbo0CM