I downsized from a 5 bed to a 3 bed. Fitting it all in was a bit of headache. However, you tend to fill the space you have. So after some really brutal hours with a skip, I managed.
You have to first get rid of the things you don't really need (actually surprisingly easy); then you have to get rid of the things you don't really want; then you have to get rid of the things that "might come in useful". Seriously, I crunched a 5 bed farmhouse into a 3 bed terrace. Although I then did re-expand into a 5 bed.......